A few weeks ago, a technician installed Windows 7 on a laptop I had, as a good turn (some good turn) and I was damned if I could locate my portable hard drives on it when I connected them to my PC - even installing windows 7 from my own windows 7 disk, did not resolve the problem and I thought my above technician installed Windows PC, was stuffed.
However, it turned out to be an easy fix, once I worked out the procedures:
Right click Computer icon
Select Manage which puts you in Computer Management (local).
In Storage select Disk Management
Double click Disk Management and you will see all of the hard drives connected to your PC
Select the portable hard drive which does not have a prefix (B-Z and add a letter (say H)).
When you check your hard drives directory your previously invisible H drive is now available and can be used.
Sometimes hard drives clash and your portable hard drive is "offline" - you can see this on the left side of the monitor screen.
Just double click on "offline" to return the drive to "online" and over ride your PC's earlier command.