I think our #Brother printer is messing up my FAT32 flash drive. Several time I formatted it, but then after using it on this printer there would be issues, sometimes my #Debian laptop refused to mount and read, sometime #Windows failed to detect filesystem or failed to read/write files properly. But the thing to to keep it universal for Windows, #Linux, #BSD and the printer, FAT32 seems to be the only choice. I am gonna use another flash drive for now just to make sure there is no malfunction in the thumb driver itself.

Does the printer offer a way to dismount the USB before you pull it? I would guess not, because it’s a printer; it’s a nonzero chance an issue with the printer is causing it to keep reading the drive after the print job, while you pull the drive. Try with another USB and see if the same thing happens; if you can, try the original USB with a different printer as well.