While browsing on a new found blog, the Swiss Army Librarian (aka Brian Herzog), I learned something I didn't know about saving files to a Flash Drive. Most folks will never have this problem, but people who save lots of small files to their flash drive might. It turns out that there is a limit to the number of files a root directory can hold. The limit is pretty high - about 700 or so - but once you hit it, you can't save any more files to that drive (or disk) no matter how much space you have left. So if you save lots of small files, be sure to set up some folders and save your files into those, instead of in the main directory. Otherwise, you might be seeing a file permission error that suggests your drive is "full, write-protected, or damaged."
Blog post by the Swiss Army Librarian
CNET forums post on same topic