Once the archive verifies, USB detection runs (so a stick plugged in while
the backup ran is found): mounted removable storage is auto-detected via
lsblk -J + a recursive jq filter (rm, mounted, type part|disk — JSON makes
spacey mountpoints safe), or BACKUP_USB_ROOT pins a fixed stick and skips
detection. None mounted → one re-scan prompt ('s' skips, EOF from cron
skips silently, rc stays 0); one stick → y/N confirm; several → numbered
pick (0 = skip). The copy lands in <usb>/backups/ (mkdir -p, chmod 600
best-effort — a vfat chmod failure warns, never fails the copy) and the
transfer is proven 100% by sha256 source-vs-copy before any success is
announced; a mismatch warns with both hashes, notifies 'USB copy FAILED',
and exits 1. The ERR trap is re-armed before the USB phase so a copy
failure no longer notifies 'Backup FAILED'. Docs: usage() Environment,
POS.md backup row, howto/system.md (USB section + env table + mismatch
troubleshooting), DEV.md system.env list. Stub suite
(/tmp/opencode/backup-test, HOME-isolated, sudo/gpg/lsblk/sender stubs,
corrupting-cp + vfat-chmod overrides, per-test lsblk JSON fixtures):
40/40 green.
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