I do a lot of personal work on my computer at work, searching the internet and downloading stuff.
I've had problems with USB flashdrives from time to time not showing up as a storage drive in My Computer, while if you go to:
Control Panel / System / Hardware / Device Manager / USB Controllers
the device is listed as present and working.
When that's the situation, it sometimes works to disable the device, unplug the USB drive from the computer, and then plug it back in. (You might have to go back to USB Controllers and re-enable the device.)
Anyway, I just had a problem with a hard drive plugged into a Vantec Hard Drive Docking station that was plugged into a USB port. Again, the hard drive was present in Device Manager under Disk Drives, and USB mass storage controller.
I was able to enable this hard drive by re-assigning a new Drive Letter to the drive:
Control Panel / Administrative Tools / Computer Management / Storage / Disk Management
If the drive appears in the list select, click to highlight the appropriate drive.
From the Menu Bar select: Action
A drop down list opens and select: All Tasks
Change Drive Letter and Paths
In the box, select a new letter to assign to the drive, and then click OK, and exit.