|Hello, when you connect your external hdd, do you feel any vibrations?|
If yes, do you have same error when you connect it to other PC's?
Please open disk management and see if you can find your drive here.
If yes, it may be unmounted. You can refer here about assigning drive letters: http://windows.microsoft.com/en-us/...
Finally, you can use a data recovery software (if it can be detected) this is your last hope.
Seagate File Recovery: http://www.mediafire.com/download/a...
If it is not detected, please see a local technician to do it for you.
Hope it helps.