Just had a problem with getting my Iomega USB hard drive to be recognized by Windows 7.

Actually, the first time I plugged it in it did show up, but then something unrelated happened, something crashed and the drive refused to ever mount again.

I opened up the Device Manager (Start -> Control Panel -> Hardware and Sound -> Device Manager) and then expanded the Disk drives section. It looked like this:

I guessed the one marked USB was my external drive, right-clicked and hit Uninstall

I then unplugged me external drive, waited a couple seconds and plugged it back in. Windows automatically reinstalled this driver and now everything works properly.