Cross-device flash (e.g. USB→NVMe) leaves the freshly-flashed kernel on the internal disk but the firmware will still pick the USB first at the next boot unless the stick is pulled. Previously the screen just showed a static "remove USB before rebooting!" notice and still accepted 'y', so it was easy to reboot onto the stale kernel. Now the y/enter shortcut is no-op'd (with a negative-feedback tone) while the boot USB is still enumerated in flashTargets. The warning blinks between amber (unplug USB) and red (reboot blocked). Since flashTargets re-enumerates every frame, the moment the stick is pulled the warning flips to a green "✓ USB removed, safe to reboot" and the shortcut unlocks — no polling, no stale state. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>