Originally Posted by emen

I'm 100% sure it's a bug (probably Android in general), since I've read elsewhere some HTC Android phones exhibited the same behavior. It happened twice after a complete reset (once immediately after, so without any additional, third party apps). The same USB sticks work fine for some time, after I reformat them and add some files, but problem is that it just randomly deletes my files, I never know when it will happen, sometimes everything works fine, sometimes I lose all my data. Both of my USB stick work great on my PC's, they're not some Chinese fake drive.

I believe the solution lies in update to some more recent Android version, and just wanted to warn you all to be extremely careful and to back up your data, and not to store anything important on your USB flash drives.