I use thumb flash drives quite a lot moving between computers. It is still much faster to load a gig or two from the flash drive than to use Skydrive or other "cloud" alternative. For the most part it is painless - stick it in and OS will find and mount it, no setup requires. But one day perfectly good USB drive stopped working all of the sudden on one specific computer. It works fine on the others and other USBs work fine on that computer too. What's going on?
I never got any errors and little icon indicated that it is plugged in and works fine. But it would never show up in the file explorer. A bit mystified, I went to disk management (right-click "my computer" -> manage) and found that it is indeed working fine. But there is a small problem: it uses same drive letter as one of the network shares mounted on this box.
Wow. That is interesting. From what I understood, when you use flash drive for the first time Windows runs a setup and assigns drive letter to it. Then it will just re-use it to speed things up. And if in the meantime you mount something else to this drive letter and then try to use that specific USB flash drive again - you pretty much screwed. Windows will never tell you about conflict, it will just give up on mounting this drive and walk away in silence.
It is easy to fix by changing drive letter for any of conflicting devices, you can do it in the device manager itself by right-clicking the drive and selecting "change drive letter". In my case, I've changed it to "K". No one needs "K", right? Should be safe now.