Before I reveal the solution to this vexing problem, Thomas, a quick slap on the wrist: you didn’t give me nearly enough information in your initial e-mail. In fact, it took two follow-up e-mails for me to get all the details I needed to find a fix.
So to you and all other readers, I kindly ask that you revisit my post on how to ask for tech help, and to remember that you’ll greatly increase your chances of a response if you follow those simple guidelines.
As it happens, Microsoft has a fix for the problem of unrecognized USB flash drives (and other external drives), which is most likely the result of “corrupted or incorrect keys in the registry.”
Just scroll down a bit until you see the Microsoft Fix It button, click it, then follow the instructions. That should do the trick!