HD external drive doesn't show up

[Nhoc Sin]

Hi, I'm new to this forum. I have a problem with my HDD that I'd like to get some opinions on. I plugged my HDD into the computer and it didn't show up on My Computer, but when I opened Device Manager, the HDD is listed under there. I tried another HDD but the same thing happened. Is it because my computer is corrupted?

 

[maxseven]

What does it mean "plugged my HDD" i.e. IDE/PATA internal, SATA internal, eSATA external, USB???

How is the Disk shown under Disk Management? Right-click My Computer, Manage, Storage>Disk Management. Assuming the disk shows-up with a Healthy partition, can you right-click and assign a drive letter to it?