Size limitation for thumbdrives on xbmc?

I am using a homemade usb cable that is attached to my xbox and my xbox is not networked. I have to move it to another room to plug it up to my computer to ftp, this is why i use my 4 gig thumbdrive to put tv shows and movies on quickly. You can watch them right off the thumbdrive then delete and refill as you want from ur computer. I know a 4 gig works and I know trying to use a external hd wont work like this. I bought that 8gig hoping but no luck as of yet. Anyone know a fix or a for sure size limitation?????