If you have a USB hub (or a USB device that includes a USB hub) connected to your computer, and you plug a flash drive or other USB device into the hub and the flash drive doesn't get recognized and mounted by Windows, try this: plug the flash drive into a USB slot on your computer instead of plugging it into the hub.
This kind of problem has happened to me often with laptops, which often seem to underpower their USB slots for some reason. I even had a situation once where a USB mouse plugged into a USB slot on the back of the laptop wouldn't work, but when I plugged the mouse into a slot on the side of the machine, it was recognized. And it can also happen frequently with laptop port replicators in docking stations.
So when in doubt, avoid plugging USB devices into USB hubs or port replicators—plug them directly into slots on your computer if possible.
If you have feedback concerning this tip, I'd love to hear from you.