This is not necessarily pin point specific to your particular situation, but might be, or could be useful to others finding their way into this thread.
Unfortunately, there are various flash memory products available on the market which are fraudulently represented. They use various methods to appear at first blush to be the advertised product and capacity while actually being far less. These methods often/usually can and will cause trouble at some point; sooner or later. This trouble often shows itself as strange errors and behaviors that users waste precious time trying to troubleshoot.
The best way I know to avoid fake products, is to buy trusted name brands from trusted retailers. For example, 3rd party jimmybob on Amazon or ebay might not be the best place to get your flash memory products from, especially if the price seems a little bit towards too good.
If you wish to verify flash media as legit or not, I suggest some free software called F3. It's available for most Linux distros right in the standard repos. Since it is FOSS, it may even be available for Win or Mac but I don't know. the F3 comes with a few programs. The f3probe program is the one to check with.
I hope this helps someone out.
github.com
