Refreshes the USB disk (sda2) as a live boot-fallback clone of the
running root, same self-sync guard/excludes/rsync flags as the SD
script. Fixed copy-paste leftovers from sync-sd-backup.sh (SD_ROOT_PART,
mount point, and log message all still said "SD").
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YJfEELeh3ercpRBp8yYrYS
