Hi guys, I know this is an old thread but I have a solution that works for me, and rather than piss off with my working hard drive I'm going to actually post my solution. Also this thread is the top result in google.
I bought a new 3TB external USB 3.0 drive (Seagate "Expansion"). Plugged it in to a USB 2.0 port, windows saw it and installed it. It showed up in device manager under "disk drives" as "Seagate Exapansion Desk USB device", so that all seems fine. However, it didn't show up or appear in explorer.
It did appear in Computer Management (Disk management) as a new disk, in the lower pane only. But it didn't appear to have a file system, and it wasn't assigned a drive letter. This is similar to other posts in this thread. I tried to assign it a drive letter, but it just threw up an error. I tried to reformat it, and again it just threw up another error. I tried different USB ports, I tried USB 3.0 ports, with the same results.
However, it was easily fixed. All I did was uninstall the device in device manager (right click, uninstall). Then I unplugged the USB cable, reinserted the USB cable into the same port, windows found it and installed it again (this process seemed identical to the first time round) but this time it showed up in explorer straight away and has a drive letter.
So I'm guessing something went wrong with the original installation, giving windows a second chance solved the issue. Good old windows....