Something to remember when copying or syncing a large number of files from a PC to a thumb drive or USB portable formatted to FAT or FAT-32: If the copying or syncing from the PC is done directly to the root of the thumb drive or USB drive, the root will quickly fill up and some files will be lost.
The way around that is to create folders at the root level and copy to the folders instead. BTW, this is a very common error - have had it happen to me also.
Regards.