Here is how I test USB sticks, MP3 players, etc bought from China on eBay. 1. Determine the reported size of the device 2. Create a file of random bytes (/dev/urandom) close to that size. 3. Copy to stick 4. md5sum both the file on your hard disk and the one on the stick. If it matches the the size reported is the size of the flash. Is there a better way?