Magical Unchanging Flash Storage
This is a caching issue with the kernel's file system driver. I've seen issues like this crop up before and it's almost certainly that. You need to report a bug and help a developer to discover what is wrong/special with your system in order to get a fix.
https://help.ubuntu.com/community/ReportingBugs
For now, check if you can write to the usb sticks using the Ubuntu LiveCD. Determine what would happen if you were to do a fresh install and then do a fresh install if you need to.