Next time, if there is a next time, see if Disk Utility or System Information can tell you anything about the format used on the thumb drive, FAT16, FAT32 (MS-DOS), or NTFS and what Partition Map Scheme. I don't have a FAT16 volume around to test with but I would bet it was FAT16. FAT16 has significant limits on file size and the number of files in the top level directory. While I can find nothing official
this apple discussion thread suggests FAT16 is no longer supported and I know Apple has not supported formatting a disk in FAT16 for a good number of years.
