I suspect, as you apparently do, that it is hardware-dependent. It does have ext2 on it. It is a Patriot Rage 8GB drive, supposedly the fastest flash drive available.I must have tried 5.1.1 on 2 dozen or more machines and never saw that Paul. (many of those with the same usb key installation). What filesystem is on the key? I usually use ext2. I know many prefer FAT for windows compatibility. Filesystem may not even be the problem. Some things have been changed in the init script since 431. If can you try another key, let us know if it still happens.
I went into the icon manager and told it to redraw the drive icons, but that didn't do anything.
This is an annoyance, but not very important. I'd rather dig into Puppy Package Manager which needs attention and is very important.