How can I test if a USB thumb drive is fake?
Its been known to happen - most likely the drive has modified firmware or a modified fat table showing the wrong size. I'd image the drive for fun, then wipe the drive and reformat to get its actual size -if its that, its the fat table. Else it may be firmware
Simple Checklist:
Device listed on eBay
Price is too good to be true
Pry it open, read the chip number, and look it up.
You could do this by having a particular sized disc, and formatting it to whatever you want the flashdrive to show. Then copy the first 512bytes with dd from the hard drive to the USB. The USB should report being huge now! D: I'll try it later.
If the drive is formatted to FAT32, then the largest size for a single file is (about) 4GB. Try reformatting it to a different file system, like NTFS.