Fix cross-drive backup failing for system-drive destinations. The
/mnt/hdd_placeholder folder is on the internal SSD, so IsMountPoint()
returned false and the old code hard-blocked it.
- ValidateDestination() (crossdrive.go): onSystemDrive flag replaces
boolean-only check; system drives require ≥10 GB free and <90% usage
to protect OS stability; external drives just need ≥100 MB free.
- CheckBackupDestination() (mounts_linux.go): Tier 4 now branches on
h.SystemDrive; system drive blocks at <10 GB free or ≥90% usage
(matches runner enforcement); external drive unchanged (warn 90%,
block 95%). Removes the && h.Severity == "ok" guard that was
preventing system-drive critical messages from overriding warnings.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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