Couple things:

1 - you’ve booted the router with the flash-drive inserted, right? I think there’s a disconnect between the kernel recognizing flash devices (which happens asynchronously) vs. when the UI does (which IIRC, only happens at boot time)

2 - you have to really go out of your way to brick one of these units; I flashed the completely wrong FW once and was able to unbrick just fine