Recently I came upon a problem where Vista wouldn’t show my external Hard Drive as a drive letter, it was kind frustrating because when I powered it on, the computer showed that something was being installed.
so I just GOOGLED it. This only brought up fragmented questions and answers … maybe I should have Binged it LOL.  Anyway here is what I found and what worked for me

First turn off the external Hard drive and Open up your Device Manager Via the Control panel.
Browse to the Universal Serial Bus Controller and expand the menu to see the devices.
You might want to maximize the window so you can see all the devices in the menu. Note: You need to note all the devices in the menu this is the key otherwise you will have to start all over.
Now power on the External Drive. Once the Device manager has been refreshed look for the device that has just been installed.

The Driver that was installed on my computer was oddly a camera driver! kinda odd huh. Well I had installed this camera about a month ago, you might have seen this type of camera before, the type that doesn’t use mass storage… its better known for being a e El cheapo camera with less than QVGA specs. and the type you get at the dollar store for 5-10 bucks.

Well reader do you want to know how I made the external hard drive work again? Ok I’ll tell you. All I did is right click on the device that was just installed and clicked “Update Driver Software” Next you will get another screen click “Browse my computer for driver software” after that Choose “Let me pick from a list of device drivers on my computer” and select the “USB Mass Storage Device” Driver And POW it will work I hope this works for you, and have fun with your Hard Drive that has Come Back to life :D