diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3ebf6b4..de21604 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,37 @@
 ## Changelog
 
+### v0.66.0 — userdata layout + shared-storage ownership convention (2026-06-14)
+
+Customer-facing `userdata/` tree (sibling of appdata/backups under each drive's felhom-data namespace)
+with a shared-ownership convention so FileBrowser + content apps collaborate without permission
+collisions. Spike: `felhom.eu/documentation/audits/SPIKE-userdata-layout-2026-06-14.md`. Pairs with the
+app-catalog commit that repoints media mounts to `${USERDATA_PATH}`.
+
+- **Convention helper** (`internal/appbackup/userdata.go`): `EnsureUserdataDir`/`EnsureDirOwned` =
+  MkdirAll → explicit `Chmod(ModeSetgid|0775)` (MkdirAll's mode is umask-masked AND drops setgid) →
+  chown group to `SharedContentGID` (1000). `UserdataDir`, `UserdataSkeleton` (media/{movies,tv,music,
+  audiobooks,books,comics,photos}, downloads, import/{paperless,calibre}, roms, documents),
+  `EnsureUserdataSkeleton`. Linux chown via `chownGID`/`StatGID` (`userdata_linux.go`); no-op stub
+  off-Linux (`userdata_other.go`).
+- **USERDATA_PATH injection** (`stackEnv`, manager.go): injects `USERDATA_PATH = <HDD_PATH>/userdata`
+  (HDD_PATH is the namespace root) alongside HDD_PATH, so the catalog's `${USERDATA_PATH}/...` mounts
+  resolve.
+- **Skeleton pre-create**: `registerStoragePath` + `syncFileBrowserMounts` ensure the full skeleton on
+  every storage path (system + additional drives) with the convention.
+- **Deploy belt**: `composeExecCustomEnv` (gated on `up`) pre-creates every `${USERDATA_PATH}/...`
+  bind source the stack declares (`ParseComposeUserdataMounts` + `ensureUserdataMounts`) so Docker
+  never auto-creates a userdata dir as guest-root — covers apps not in the skeleton.
+- **FileBrowser mount switch** (`syncFileBrowserMounts`): mounts `<drive>/userdata` (was `appdata`) →
+  `/srv/<name>`. FileBrowser runs as uid 1000 → can now create folders + upload into the 2775 setgid
+  userdata (fixes the permission-denied); app internals (appdata/) are no longer browsable.
+- **#8 migration fix** (`migrate.go`): the non-app merge walk now preserves the SOURCE dir's full mode
+  (incl. setgid via `preserveDirOwnership`) + group, and `copyFile` preserves the full file mode
+  (`fi.Mode()`, not `.Perm()`) + group — so the ownership convention survives a whole-drive `MigrateAll`.
+- Non-hollow tests: `EnsureDirOwned` produces 02775+setgid+gid (Linux companion proves a plain MkdirAll
+  has NO setgid); skeleton structure; `ParseComposeUserdataMounts` selectivity; deploy belt creates the
+  declared dirs; **migration preserves setgid+group** (Linux; mutation-proven against the pre-fix
+  0755/.Perm() path).
+
 ### v0.65.0 — data migration + self-serve decommission (B1+B2) (2026-06-14)
 
 Customer-self-serve storage **migration** (move app data between drives) and **decommission** (retire
diff --git a/controller/internal/appbackup/derivestackname_test.go b/controller/internal/appbackup/derivestackname_test.go
index 0553092..c1c001f 100644
--- a/controller/internal/appbackup/derivestackname_test.go
+++ b/controller/internal/appbackup/derivestackname_test.go
@@ -14,8 +14,8 @@ func TestDeriveStackName_KnownCrossRef(t *testing.T) {
 		note      string
 	}{
 		{"romm-postgres", "romm", "role suffix of a real stack → strip"},
-		{"my-cache", "my-cache", "container name IS a real stack → do NOT strip to 'my'"},     // pre-fix: "my"
-		{"my-cache-postgres", "my-cache", "strip role, result is a known stack"},               // pre-fix: "my-cache" (ok) — but via prefix here
+		{"my-cache", "my-cache", "container name IS a real stack → do NOT strip to 'my'"}, // pre-fix: "my"
+		{"my-cache-postgres", "my-cache", "strip role, result is a known stack"},          // pre-fix: "my-cache" (ok) — but via prefix here
 		{"paperless-ngx-postgres", "paperless-ngx", "multi-hyphen stack, role suffix"},
 		{"romm_postgres", "romm", "underscore-separated compose name → longest known prefix"},
 		{"romm-1", "romm", "compose numeric suffix → longest known prefix"},
diff --git a/controller/internal/appbackup/userdata.go b/controller/internal/appbackup/userdata.go
new file mode 100644
index 0000000..ecd2668
--- /dev/null
+++ b/controller/internal/appbackup/userdata.go
@@ -0,0 +1,76 @@
+package appbackup
+
+import (
+	"os"
+	"path/filepath"
+)
+
+// Customer-facing userdata layout + the shared-storage ownership convention (v0.66.0).
+//
+// userdata/ is a sibling of appdata/ and backups/ under a drive's felhom-data namespace. It is the
+// ONLY customer-browsable tree (FileBrowser mounts it). Apps that handle customer content write here.
+//
+// Ownership convention: every userdata dir is group-owned by SharedContentGID, mode 2775 (setgid +
+// group-rwx). Setgid makes new files/dirs inherit the shared group regardless of which app (or
+// FileBrowser) created them, so members collaborate without permission collisions. FileBrowser
+// (uid/gid 1000) and the content apps (PUID/PGID 1000, or pinned user 1000:1000) are all members.
+
+// SharedContentGID is the group that owns the userdata tree.
+const SharedContentGID = 1000
+
+// userdataDirMode is the on-disk mode for every userdata dir: setgid + group-rwx. os.ModeSetgid (NOT
+// the raw 0o2000) is how Go's Chmod requests S_ISGID. MkdirAll's mode is umask-masked AND drops the
+// setgid bit, so an explicit Chmod is mandatory after MkdirAll.
+const userdataDirMode = os.ModeSetgid | 0o775
+
+// UserdataDir returns the customer-facing userdata root under a namespace root.
+func UserdataDir(nsRoot string) string {
+	return filepath.Join(nsRoot, "userdata")
+}
+
+// UserdataSkeleton is the standard subtree created on every storage path (relative to UserdataDir).
+// ASCII, no spaces (flows through ${} interpolation, shell, and the rsync merge walk).
+func UserdataSkeleton() []string {
+	return []string{
+		"media", "media/movies", "media/tv", "media/music", "media/audiobooks",
+		"media/books", "media/comics", "media/photos",
+		"downloads",
+		"import", "import/paperless", "import/calibre",
+		"roms",
+		"documents",
+	}
+}
+
+// EnsureDirOwned creates path (idempotent) and enforces the convention: mode 2775 via an explicit
+// Chmod incl. setgid (MkdirAll cannot) + group = gid. Setting an arbitrary group needs CAP_CHOWN —
+// the in-guest controller runs as root, so this succeeds in production. Returns the first hard error.
+func EnsureDirOwned(path string, gid int) error {
+	if err := os.MkdirAll(path, 0o755); err != nil {
+		return err
+	}
+	if err := os.Chmod(path, userdataDirMode); err != nil {
+		return err
+	}
+	return chownGID(path, gid)
+}
+
+// EnsureUserdataDir applies the convention with the shared content group (GID 1000). Idempotent.
+func EnsureUserdataDir(path string) error { return EnsureDirOwned(path, SharedContentGID) }
+
+// EnsureUserdataSkeleton creates the full userdata tree under a namespace root with the convention.
+// It creates ALL dirs even if one errors (so a single chown/chmod hiccup doesn't truncate the tree),
+// returning the first error seen for the caller to log.
+func EnsureUserdataSkeleton(nsRoot string) error {
+	base := UserdataDir(nsRoot)
+	var firstErr error
+	rec := func(e error) {
+		if e != nil && firstErr == nil {
+			firstErr = e
+		}
+	}
+	rec(EnsureUserdataDir(base))
+	for _, sub := range UserdataSkeleton() {
+		rec(EnsureUserdataDir(filepath.Join(base, sub)))
+	}
+	return firstErr
+}
diff --git a/controller/internal/appbackup/userdata_linux.go b/controller/internal/appbackup/userdata_linux.go
new file mode 100644
index 0000000..7eb1c68
--- /dev/null
+++ b/controller/internal/appbackup/userdata_linux.go
@@ -0,0 +1,20 @@
+//go:build linux
+
+package appbackup
+
+import (
+	"os"
+	"syscall"
+)
+
+// chownGID sets the GROUP of path (owner unchanged via -1). Setting an arbitrary group requires
+// CAP_CHOWN; the in-guest controller runs as root, so this succeeds in production.
+func chownGID(path string, gid int) error { return os.Chown(path, -1, gid) }
+
+// StatGID returns the owning GID of fi (Linux). ok=false when the underlying stat is unavailable.
+func StatGID(fi os.FileInfo) (int, bool) {
+	if st, ok := fi.Sys().(*syscall.Stat_t); ok {
+		return int(st.Gid), true
+	}
+	return -1, false
+}
diff --git a/controller/internal/appbackup/userdata_other.go b/controller/internal/appbackup/userdata_other.go
new file mode 100644
index 0000000..4c4a2ea
--- /dev/null
+++ b/controller/internal/appbackup/userdata_other.go
@@ -0,0 +1,11 @@
+//go:build !linux
+
+package appbackup
+
+import "os"
+
+// chownGID is a no-op off Linux (the dev machine has no POSIX group ownership). Production is Linux.
+func chownGID(path string, gid int) error { return nil }
+
+// StatGID is unavailable off Linux.
+func StatGID(fi os.FileInfo) (int, bool) { return -1, false }
diff --git a/controller/internal/appbackup/userdata_setgid_linux_test.go b/controller/internal/appbackup/userdata_setgid_linux_test.go
new file mode 100644
index 0000000..545af87
--- /dev/null
+++ b/controller/internal/appbackup/userdata_setgid_linux_test.go
@@ -0,0 +1,43 @@
+//go:build linux
+
+package appbackup
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+// TestEnsureDirOwned_Setgid is the load-bearing assertion: EnsureDirOwned produces a dir with the
+// SETGID bit + group-rwx (mode 02775) and the requested group. Uses the test's own gid so the chown
+// succeeds without root. Companion: a plain MkdirAll(0755) does NOT get setgid — proving the explicit
+// Chmod is what sets it (the spike's collision fix). This test FAILS on a pre-fix MkdirAll-only impl.
+func TestEnsureDirOwned_Setgid(t *testing.T) {
+	gid := os.Getgid()
+	dir := filepath.Join(t.TempDir(), "userdata", "media", "movies")
+	if err := EnsureDirOwned(dir, gid); err != nil {
+		t.Fatalf("EnsureDirOwned: %v", err)
+	}
+	fi, err := os.Stat(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if fi.Mode()&os.ModeSetgid == 0 {
+		t.Errorf("dir is missing the setgid bit: mode=%v", fi.Mode())
+	}
+	if perm := fi.Mode().Perm(); perm != 0o775 {
+		t.Errorf("dir perm = %o, want 0775", perm)
+	}
+	if g, ok := StatGID(fi); !ok || g != gid {
+		t.Errorf("dir gid = %d (ok=%v), want %d", g, ok, gid)
+	}
+
+	// Companion: the pre-fix behaviour (MkdirAll only, no explicit setgid Chmod) → NO setgid.
+	plain := filepath.Join(t.TempDir(), "plain")
+	if err := os.MkdirAll(plain, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if pfi, _ := os.Stat(plain); pfi.Mode()&os.ModeSetgid != 0 {
+		t.Errorf("plain MkdirAll unexpectedly has setgid — the explicit Chmod is not load-bearing")
+	}
+}
diff --git a/controller/internal/appbackup/userdata_test.go b/controller/internal/appbackup/userdata_test.go
new file mode 100644
index 0000000..9ab5898
--- /dev/null
+++ b/controller/internal/appbackup/userdata_test.go
@@ -0,0 +1,52 @@
+package appbackup
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+// TestSharedContentGID pins the shared content group to 1000 (FileBrowser's gid + the apps' PUID/PGID).
+func TestSharedContentGID(t *testing.T) {
+	if SharedContentGID != 1000 {
+		t.Errorf("SharedContentGID = %d, want 1000", SharedContentGID)
+	}
+}
+
+// TestUserdataSkeleton_List asserts the locked skeleton subdir set.
+func TestUserdataSkeleton_List(t *testing.T) {
+	got := map[string]bool{}
+	for _, s := range UserdataSkeleton() {
+		got[s] = true
+	}
+	for _, want := range []string{
+		"media/movies", "media/tv", "media/music", "media/audiobooks", "media/books",
+		"media/comics", "media/photos", "downloads", "import/paperless", "import/calibre",
+		"roms", "documents",
+	} {
+		if !got[want] {
+			t.Errorf("skeleton missing %q", want)
+		}
+	}
+}
+
+// TestUserdataDir confirms the userdata root is a sibling under the namespace.
+func TestUserdataDir(t *testing.T) {
+	if got := UserdataDir("/mnt/felhom-usb"); got != filepath.Clean("/mnt/felhom-usb/userdata") {
+		t.Errorf("UserdataDir = %q", got)
+	}
+}
+
+// TestEnsureUserdataSkeleton_Structure: every skeleton dir is created (chown may fail off-root, which
+// is ignored — dirs + setgid still land). Runs cross-platform.
+func TestEnsureUserdataSkeleton_Structure(t *testing.T) {
+	ns := t.TempDir()
+	_ = EnsureUserdataSkeleton(ns) // ignore chown error on a non-root CI host
+	base := UserdataDir(ns)
+	for _, sub := range append([]string{""}, UserdataSkeleton()...) {
+		p := filepath.Join(base, sub)
+		if fi, err := os.Stat(p); err != nil || !fi.IsDir() {
+			t.Errorf("skeleton dir missing: %s (%v)", p, err)
+		}
+	}
+}
diff --git a/controller/internal/stacks/delete.go b/controller/internal/stacks/delete.go
index 192fa16..24c28d5 100644
--- a/controller/internal/stacks/delete.go
+++ b/controller/internal/stacks/delete.go
@@ -62,10 +62,10 @@ func ProtectedHDDPaths(hddPath string) map[string]bool {
 	return map[string]bool{
 		// Model A: the in-guest drive mount IS the felhom-data namespace root, so backups/ and
 		// appdata/ sit directly under it (no felhom-data segment).
-		hddPath:                               true,
-		filepath.Join(hddPath, "appdata"):     true,
-		filepath.Join(hddPath, "backups"):     true,
-		filepath.Join(hddPath, "media"):       true,
+		hddPath:                                true,
+		filepath.Join(hddPath, "appdata"):      true,
+		filepath.Join(hddPath, "backups"):      true,
+		filepath.Join(hddPath, "media"):        true,
 		filepath.Join(hddPath, "Dokumentumok"): true,
 		// Legacy pre-Model-A double-nest location; kept protected so any leftover data there is
 		// never wiped by a removal.
@@ -505,6 +505,53 @@ func buildPathInfo(path string) HDDPath {
 	return item
 }
 
+// ParseComposeUserdataMounts reads a docker-compose.yml and extracts the host bind-source paths that
+// reference ${USERDATA_PATH} (resolved to userdataPath) — the dirs the deploy belt must pre-create
+// with the userdata convention. Same scanner shape as ParseComposeHDDMounts.
+func ParseComposeUserdataMounts(composePath, userdataPath string) []string {
+	if userdataPath == "" {
+		return nil
+	}
+	data, err := os.ReadFile(composePath)
+	if err != nil {
+		return nil
+	}
+	var mounts []string
+	seen := make(map[string]bool)
+	scanner := bufio.NewScanner(strings.NewReader(string(data)))
+	inVolumes := false
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if strings.HasPrefix(line, "volumes:") {
+			inVolumes = true
+			continue
+		}
+		if inVolumes && !strings.HasPrefix(line, "-") && !strings.HasPrefix(line, "#") && line != "" {
+			inVolumes = false
+		}
+		if !inVolumes || !strings.HasPrefix(line, "- ") {
+			continue
+		}
+		mountStr := strings.Trim(strings.TrimPrefix(line, "- "), "\"'")
+		parts := strings.SplitN(mountStr, ":", 3)
+		if len(parts) < 2 {
+			continue
+		}
+		hostPath := strings.ReplaceAll(parts[0], "${USERDATA_PATH}", userdataPath)
+		cleanPath := filepath.Clean(hostPath)
+		cleanUD := filepath.Clean(userdataPath)
+		// must be userdataPath itself or a subpath (clean before check — traversal-safe)
+		if cleanPath != cleanUD && !strings.HasPrefix(cleanPath, cleanUD+string(filepath.Separator)) {
+			continue
+		}
+		if !seen[cleanPath] {
+			seen[cleanPath] = true
+			mounts = append(mounts, cleanPath)
+		}
+	}
+	return mounts
+}
+
 // ParseComposeHDDMounts reads a docker-compose.yml and extracts host paths
 // that reference the HDD path from volume bind mounts.
 func ParseComposeHDDMounts(composePath, hddPath string) []string {
diff --git a/controller/internal/stacks/deploy.go b/controller/internal/stacks/deploy.go
index 08341ec..e8d12e4 100644
--- a/controller/internal/stacks/deploy.go
+++ b/controller/internal/stacks/deploy.go
@@ -919,4 +919,4 @@ func randomAlphanumeric(length int) (string, error) {
 		result[i] = alphanumChars[n.Int64()]
 	}
 	return string(result), nil
-}
\ No newline at end of file
+}
diff --git a/controller/internal/stacks/deploy_crashsafety_regression_test.go b/controller/internal/stacks/deploy_crashsafety_regression_test.go
index 384f30c..8964619 100644
--- a/controller/internal/stacks/deploy_crashsafety_regression_test.go
+++ b/controller/internal/stacks/deploy_crashsafety_regression_test.go
@@ -83,7 +83,7 @@ func TestTransitionalDeployStateReadsNotDeployed(t *testing.T) {
 	}
 	// This is exactly the expression ScanStacks uses: deployed := cfg != nil && cfg.Deployed
 	if got.Deployed {
-		t.Fatalf("CTRL-T2-1: a deploy that did not complete reads as Deployed=true — ghost-deployed; "+
+		t.Fatalf("CTRL-T2-1: a deploy that did not complete reads as Deployed=true — ghost-deployed; " +
 			"DeployStack must persist Deployed:false until compose succeeds")
 	}
 
diff --git a/controller/internal/stacks/manager.go b/controller/internal/stacks/manager.go
index ffcb8dc..8ae336e 100644
--- a/controller/internal/stacks/manager.go
+++ b/controller/internal/stacks/manager.go
@@ -13,6 +13,7 @@ import (
 	"sync"
 	"time"
 
+	"gitea.dooplex.hu/admin/felhom-controller/internal/appbackup"
 	"gitea.dooplex.hu/admin/felhom-controller/internal/config"
 	"gitea.dooplex.hu/admin/felhom-controller/internal/crypto"
 	"gitea.dooplex.hu/admin/felhom-controller/internal/settings"
@@ -122,6 +123,33 @@ func NewManager(cfg *config.Config, logger *log.Logger) (*Manager, error) {
 	}, nil
 }
 
+// ensureUserdataMounts is the deploy belt: pre-create every ${USERDATA_PATH}/... bind source the
+// stack declares with the userdata convention, so Docker never auto-creates one as guest-root.
+func (m *Manager) ensureUserdataMounts(stackDir string, env []string) {
+	userdataPath := envLookup(env, "USERDATA_PATH")
+	if userdataPath == "" {
+		return
+	}
+	composePath := filepath.Join(stackDir, "docker-compose.yml")
+	for _, src := range ParseComposeUserdataMounts(composePath, userdataPath) {
+		if err := appbackup.EnsureUserdataDir(src); err != nil {
+			m.logger.Printf("[WARN] [stacks] userdata belt: ensure %s: %v", src, err)
+		}
+	}
+}
+
+// envLookup returns the value of key in a "K=V" env slice (last occurrence wins, "" if absent).
+func envLookup(env []string, key string) string {
+	prefix := key + "="
+	val := ""
+	for _, e := range env {
+		if strings.HasPrefix(e, prefix) {
+			val = e[len(prefix):]
+		}
+	}
+	return val
+}
+
 // SetEncryptionKey sets the AES-256 key used to encrypt/decrypt sensitive values in app.yaml.
 func (m *Manager) SetEncryptionKey(key []byte) {
 	m.mu.Lock()
@@ -829,6 +857,12 @@ func (m *Manager) stackEnv(stackDir string) []string {
 		for k, v := range appCfg.Env {
 			env = append(env, fmt.Sprintf("%s=%s", k, v))
 		}
+		// Inject USERDATA_PATH = <namespace root>/userdata alongside HDD_PATH (v0.66.0). HDD_PATH IS
+		// the namespace root (the chosen StoragePath: a Model-A user drive's mount, or the SSD's
+		// felhom-data dir), so the catalog's ${USERDATA_PATH}/... mounts resolve under userdata/.
+		if hdd := appCfg.Env["HDD_PATH"]; hdd != "" {
+			env = append(env, fmt.Sprintf("USERDATA_PATH=%s", appbackup.UserdataDir(hdd)))
+		}
 	}
 
 	return env
@@ -857,6 +891,13 @@ func (m *Manager) composeExecCustomEnv(dir string, env []string, args ...string)
 		cmd.Env = env
 	}
 
+	// Deploy belt (v0.66.0): before `up`, pre-create every ${USERDATA_PATH}/... bind source with the
+	// userdata convention (2775 setgid, gid 1000) so the Docker daemon never auto-creates one as
+	// guest-root — covers apps not in the skeleton too. Best-effort: a perms hiccup never blocks deploy.
+	if len(args) > 0 && args[0] == "up" {
+		m.ensureUserdataMounts(dir, env)
+	}
+
 	// Log env var keys at debug level
 	if m.isDebug() {
 		var appKeys []string
diff --git a/controller/internal/stacks/metadata.go b/controller/internal/stacks/metadata.go
index c5055c4..ea18472 100644
--- a/controller/internal/stacks/metadata.go
+++ b/controller/internal/stacks/metadata.go
@@ -21,7 +21,7 @@ type Metadata struct {
 	AppInfo        AppInfo               `yaml:"app_info" json:"app_info"`
 	OptionalConfig []OptionalConfigGroup `yaml:"optional_config" json:"optional_config"`
 	HealthCheck    *HealthCheckConfig    `yaml:"healthcheck,omitempty" json:"healthcheck,omitempty"`
-	Integrations   []IntegrationDef     `yaml:"integrations,omitempty" json:"integrations,omitempty"`
+	Integrations   []IntegrationDef      `yaml:"integrations,omitempty" json:"integrations,omitempty"`
 }
 
 // AppInfo holds detailed app information for the info page.
@@ -52,25 +52,25 @@ type OptionalConfigField struct {
 
 // ResourceHints describe what the app needs.
 type ResourceHints struct {
-	MemRequest    string `yaml:"mem_request" json:"mem_request"`
-	MemLimit      string `yaml:"mem_limit" json:"mem_limit"`
-	PiCompatible  bool   `yaml:"pi_compatible" json:"pi_compatible"`
-	NeedsHDD      bool   `yaml:"needs_hdd" json:"needs_hdd"`
-	HungarianUI   bool   `yaml:"hungarian_ui" json:"hungarian_ui"`
+	MemRequest   string `yaml:"mem_request" json:"mem_request"`
+	MemLimit     string `yaml:"mem_limit" json:"mem_limit"`
+	PiCompatible bool   `yaml:"pi_compatible" json:"pi_compatible"`
+	NeedsHDD     bool   `yaml:"needs_hdd" json:"needs_hdd"`
+	HungarianUI  bool   `yaml:"hungarian_ui" json:"hungarian_ui"`
 }
 
 // DeployField defines one configuration field shown during first deployment.
 type DeployField struct {
-	EnvVar           string          `yaml:"env_var" json:"env_var"`
-	Label            string          `yaml:"label" json:"label"`
-	Type             string          `yaml:"type" json:"type"`               // domain, subdomain, secret, password, path, text, select, boolean
-	Generate         string          `yaml:"generate" json:"generate"`       // e.g., "password:24", "hex:32", "static:admin"
-	Default          string          `yaml:"default" json:"default"`
-	Required         bool            `yaml:"required" json:"required"`
-	Placeholder      string          `yaml:"placeholder" json:"placeholder"`
-	Description      string          `yaml:"description" json:"description"`
+	EnvVar            string         `yaml:"env_var" json:"env_var"`
+	Label             string         `yaml:"label" json:"label"`
+	Type              string         `yaml:"type" json:"type"`         // domain, subdomain, secret, password, path, text, select, boolean
+	Generate          string         `yaml:"generate" json:"generate"` // e.g., "password:24", "hex:32", "static:admin"
+	Default           string         `yaml:"default" json:"default"`
+	Required          bool           `yaml:"required" json:"required"`
+	Placeholder       string         `yaml:"placeholder" json:"placeholder"`
+	Description       string         `yaml:"description" json:"description"`
 	LockedAfterDeploy bool           `yaml:"locked_after_deploy" json:"locked_after_deploy"`
-	Options          []SelectOption  `yaml:"options" json:"options,omitempty"`
+	Options           []SelectOption `yaml:"options" json:"options,omitempty"`
 	// DataKey marks a field as a DATA-ENCRYPTING key (e.g. AdventureLog's "Titkosítási kulcs"):
 	// the app encrypts stored data with it, so regenerating it would render restored data
 	// unreadable. It is a fail-closed annotation only — the recovery unit never stores secrets;
@@ -113,7 +113,7 @@ type HealthCheckConfig struct {
 
 // HealthCheckItem defines a single health check probe.
 type HealthCheckItem struct {
-	Type   string             `yaml:"type" json:"type"`                         // "http", "api", "tcp"
+	Type   string             `yaml:"type" json:"type"` // "http", "api", "tcp"
 	Port   int                `yaml:"port" json:"port"`
 	Path   string             `yaml:"path" json:"path"`                         // for http/api; default "/"
 	Method string             `yaml:"method" json:"method"`                     // for api; default "GET"
diff --git a/controller/internal/stacks/migrate.go b/controller/internal/stacks/migrate.go
index 7caaf51..4d3a757 100644
--- a/controller/internal/stacks/migrate.go
+++ b/controller/internal/stacks/migrate.go
@@ -848,7 +848,13 @@ func walkMerge(lg *log.Logger, srcNS, dstNS string, skip map[string]bool, assert
 			if assertOnly {
 				return nil
 			}
-			return os.MkdirAll(dst, 0o755)
+			if err := os.MkdirAll(dst, 0o755); err != nil {
+				return err
+			}
+			// #8 (v0.66.0): preserve the SOURCE dir's full mode (incl. setgid) + group, so the userdata
+			// ownership convention (2775 setgid, gid 1000) survives a whole-drive migration. MkdirAll's
+			// mode is umask-masked + drops setgid, so re-stamp explicitly from the source.
+			return preserveDirOwnership(dst, d)
 		}
 
 		// Symlink: recreate-if-absent (copy mode); ignored in assert mode.
@@ -994,10 +1000,15 @@ func copyFile(src, dst string) (int64, error) {
 		os.Remove(tmp)
 		return 0, err
 	}
-	if err := os.Chmod(tmp, fi.Mode().Perm()); err != nil {
+	// #8 (v0.66.0): preserve the SOURCE file's FULL mode (incl. setgid/setuid/sticky — not .Perm(),
+	// which masks them off) + group, so the userdata convention survives a whole-drive migration.
+	if err := os.Chmod(tmp, fi.Mode()); err != nil {
 		os.Remove(tmp)
 		return 0, err
 	}
+	if gid, ok := appbackup.StatGID(fi); ok {
+		_ = os.Chown(tmp, -1, gid) // best-effort; needs root for an arbitrary group (the controller is)
+	}
 	if err := os.Rename(tmp, dst); err != nil {
 		os.Remove(tmp)
 		return 0, err
@@ -1005,6 +1016,22 @@ func copyFile(src, dst string) (int64, error) {
 	return n, nil
 }
 
+// preserveDirOwnership re-stamps a freshly-created target dir with the SOURCE dir's full mode (incl.
+// setgid) and group — part of the #8 fix so the userdata convention survives a migration.
+func preserveDirOwnership(dst string, d fs.DirEntry) error {
+	info, err := d.Info()
+	if err != nil {
+		return err
+	}
+	if err := os.Chmod(dst, info.Mode()); err != nil {
+		return err
+	}
+	if gid, ok := appbackup.StatGID(info); ok {
+		_ = os.Chown(dst, -1, gid) // best-effort; root sets an arbitrary group (the controller is root)
+	}
+	return nil
+}
+
 // fileSum returns the hex sha256 of a file (streaming).
 func fileSum(path string) (string, error) {
 	f, err := os.Open(path)
diff --git a/controller/internal/stacks/migrate_setgid_linux_test.go b/controller/internal/stacks/migrate_setgid_linux_test.go
new file mode 100644
index 0000000..8280dfc
--- /dev/null
+++ b/controller/internal/stacks/migrate_setgid_linux_test.go
@@ -0,0 +1,59 @@
+//go:build linux
+
+package stacks
+
+import (
+	"io"
+	"log"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"gitea.dooplex.hu/admin/felhom-controller/internal/appbackup"
+)
+
+// TestWalkMerge_PreservesSetgid is the #8 fix proof: the non-app merge walk must preserve the SOURCE
+// dir's setgid bit + group, so the userdata ownership convention (2775 setgid, gid 1000) survives a
+// whole-drive migration. Companion: the pre-fix path (MkdirAll 0755 / copyFile .Perm()) drops setgid
+// → this test FAILS on the old code (mutation-proven separately).
+func TestWalkMerge_PreservesSetgid(t *testing.T) {
+	lg := log.New(io.Discard, "", 0)
+	src := t.TempDir()
+	dst := t.TempDir()
+
+	// Source userdata-style tree with setgid dirs (the convention).
+	media := filepath.Join(src, "userdata", "media")
+	if err := os.MkdirAll(media, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	for _, d := range []string{filepath.Join(src, "userdata"), media} {
+		if err := os.Chmod(d, os.ModeSetgid|0o775); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := os.WriteFile(filepath.Join(media, "movie.txt"), []byte("VID"), 0o664); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := walkMerge(lg, src, dst, nil, false, nil); err != nil {
+		t.Fatalf("walkMerge: %v", err)
+	}
+
+	// Target media dir must still carry setgid + the source group.
+	srcFI, _ := os.Stat(media)
+	dstFI, err := os.Stat(filepath.Join(dst, "userdata", "media"))
+	if err != nil {
+		t.Fatalf("target media dir missing: %v", err)
+	}
+	if dstFI.Mode()&os.ModeSetgid == 0 {
+		t.Errorf("migration DROPPED the setgid bit on the dir: mode=%v", dstFI.Mode())
+	}
+	if dstFI.Mode().Perm() != 0o775 {
+		t.Errorf("target dir perm = %o, want 0775", dstFI.Mode().Perm())
+	}
+	sg, _ := appbackup.StatGID(srcFI)
+	dg, _ := appbackup.StatGID(dstFI)
+	if sg != dg {
+		t.Errorf("target dir group = %d, want source group %d", dg, sg)
+	}
+}
diff --git a/controller/internal/stacks/userdata_belt_test.go b/controller/internal/stacks/userdata_belt_test.go
new file mode 100644
index 0000000..5bb3043
--- /dev/null
+++ b/controller/internal/stacks/userdata_belt_test.go
@@ -0,0 +1,73 @@
+package stacks
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"gitea.dooplex.hu/admin/felhom-controller/internal/appbackup"
+)
+
+const beltCompose = `services:
+  app:
+    image: x
+    volumes:
+      - app_config:/config
+      - ${USERDATA_PATH}/media/movies:/media/movies
+      - ${USERDATA_PATH}/downloads:/downloads
+      - ${HDD_PATH}/appdata/app:/data
+      - /etc/passwd:/host:ro
+volumes:
+  app_config:
+`
+
+// TestParseComposeUserdataMounts: only ${USERDATA_PATH}/... bind sources are returned, resolved; HDD
+// appdata mounts, named volumes, and unrelated host paths are ignored.
+func TestParseComposeUserdataMounts(t *testing.T) {
+	dir := t.TempDir()
+	cp := filepath.Join(dir, "docker-compose.yml")
+	if err := os.WriteFile(cp, []byte(beltCompose), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	ud := filepath.Clean("/mnt/felhom-usb/userdata")
+	got := map[string]bool{}
+	for _, m := range ParseComposeUserdataMounts(cp, ud) {
+		got[m] = true
+	}
+	for _, want := range []string{
+		filepath.Join(ud, "media", "movies"),
+		filepath.Join(ud, "downloads"),
+	} {
+		if !got[want] {
+			t.Errorf("missing userdata mount %q (got %v)", want, got)
+		}
+	}
+	if len(got) != 2 {
+		t.Errorf("expected exactly 2 userdata mounts, got %d: %v", len(got), got)
+	}
+}
+
+// TestEnsureUserdataMounts_CreatesBeltDirs: the deploy belt pre-creates every declared ${USERDATA_PATH}
+// bind source before compose-up (so Docker never auto-creates one as root). Uses a real temp userdata
+// root via env injection.
+func TestEnsureUserdataMounts_CreatesBeltDirs(t *testing.T) {
+	m := newMigManager(t, "") // minimal Manager (cfg+logger+settings)
+	stackDir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(stackDir, "docker-compose.yml"), []byte(beltCompose), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	ud := filepath.Join(t.TempDir(), "userdata")
+	env := []string{"USERDATA_PATH=" + ud}
+
+	// movies dir absent before
+	if _, err := os.Stat(filepath.Join(ud, "media", "movies")); err == nil {
+		t.Fatal("precondition: movies dir should not exist yet")
+	}
+	m.ensureUserdataMounts(stackDir, env)
+	for _, p := range []string{filepath.Join(ud, "media", "movies"), filepath.Join(ud, "downloads")} {
+		if fi, err := os.Stat(p); err != nil || !fi.IsDir() {
+			t.Errorf("belt did not create %s (%v)", p, err)
+		}
+	}
+	_ = appbackup.SharedContentGID // keep import referenced cross-platform
+}
diff --git a/controller/internal/web/handlers.go b/controller/internal/web/handlers.go
index 6a1292a..aa96f4e 100644
--- a/controller/internal/web/handlers.go
+++ b/controller/internal/web/handlers.go
@@ -12,6 +12,7 @@ import (
 	"strings"
 	"time"
 
+	"gitea.dooplex.hu/admin/felhom-controller/internal/appbackup"
 	"gitea.dooplex.hu/admin/felhom-controller/internal/backup"
 	"gitea.dooplex.hu/admin/felhom-controller/internal/crypto"
 	"gitea.dooplex.hu/admin/felhom-controller/internal/infra"
@@ -1456,18 +1457,19 @@ func (s *Server) syncFileBrowserMounts(resetDBOnChange bool) {
 		return
 	}
 
-	// Build volume mount lines. SCOPE to the drive's `appdata/` subtree only (Phase 4A): the customer
-	// browses their userdata, but the recovery units + Tier 2 copies under `backups/` are NOT mounted
-	// into FileBrowser at all — so the thing that restores them can't be browsed or (even read-only)
-	// surfaced. mkdir the appdata dir first so the bind source exists with sane ownership.
+	// Build volume mount lines. SCOPE to the drive's `userdata/` subtree (v0.66.0): the customer
+	// browses ONLY userdata — app internals (appdata/) and the recovery units + Tier 2 copies
+	// (backups/) are NOT mounted into FileBrowser. userdata is owned group 1000 mode 2775 (setgid),
+	// and FileBrowser runs as uid 1000 → it can create folders + upload files (the old appdata mount
+	// was guest-root 0755 → permission-denied). Pre-create the full skeleton with the convention.
 	var storageMounts []string
 	for _, sp := range paths {
 		mountName := filepath.Base(sp.Path) // "/mnt/hdd_1" → "hdd_1"
-		appdataSrc := filepath.Join(sp.Path, "appdata")
-		if err := os.MkdirAll(appdataSrc, 0755); err != nil {
-			s.logger.Printf("[WARN] [web] FileBrowser: could not ensure appdata dir %s: %v", appdataSrc, err)
+		if err := appbackup.EnsureUserdataSkeleton(sp.Path); err != nil {
+			s.logger.Printf("[WARN] [web] FileBrowser: could not ensure userdata skeleton on %s: %v", sp.Path, err)
 		}
-		line := fmt.Sprintf("      - %s:/srv/%s", appdataSrc, mountName)
+		userdataSrc := appbackup.UserdataDir(sp.Path)
+		line := fmt.Sprintf("      - %s:/srv/%s", userdataSrc, mountName)
 		storageMounts = append(storageMounts, line)
 	}
 
diff --git a/controller/internal/web/storage_handlers.go b/controller/internal/web/storage_handlers.go
index c3ce52d..f21fde9 100644
--- a/controller/internal/web/storage_handlers.go
+++ b/controller/internal/web/storage_handlers.go
@@ -219,6 +219,12 @@ func (s *Server) registerStoragePath(where, label string, setDefault bool) error
 	if strings.TrimSpace(label) == "" {
 		label = settings.InferStorageLabel(where)
 	}
+	// v0.66.0: create the full userdata skeleton with the shared-storage convention (2775 setgid,
+	// gid 1000) the moment a drive is registered — system drive AND additional drives. Idempotent;
+	// best-effort (a perms hiccup shouldn't block registration).
+	if err := appbackup.EnsureUserdataSkeleton(where); err != nil {
+		s.logger.Printf("[WARN] [web] userdata skeleton on %s: %v", where, err)
+	}
 	// Change 4: re-enrolling a previously-DECOMMISSIONED drive must un-retire it. AddStoragePath
 	// dedups a re-register into a no-op, so without this the soft marker would persist forever and the
 	// apps' "missing storage" indicator would never clear.