What brand & model of drives are they?
I am aware with some devices such as the small (mindblock what you call them, very small, flash I think) usb drives that depending on how they were originally formatted (eg: FAT), then both the number of files & space is limited. Number of files I think was the key issue last time I saw a similar issue. Question is why can you not copy 'from' those devices.
Check this article (yes it is old):
http://www.zdnet.com.au/reviews/hard...0275003,00.htm as it appears there is a password locking system.