Are these old Flash drives that have gone bad or did you just buy them?
There is a huge counterfeit flash drive market and they scam people on eBay, NYC street corners, etc because they look and are packaged like the real thing but in actuality are small sticks (like 256K, 512K, or 1GB) that have their locations mapped so that data constantly overlays the other. It makes it seem to all write fine but if you read the data back it is all gibberish.
See the article
HERE on how to test your flash drive. Unfortunately they often use cheap or defective chips in the first place and secondly, the write process has overwritten the same sectors thousands of times bringing the chip so much closer to it's RW fault limits that even if you reflash the memory to the correct specs it is a poor flash drive that won't last long; but you can see ideas how to try
HERE.