IMac won't recognise external hard drive

My iMac with OSX 10.8.4 won't recognise my external disk drive via either Firewire or USB Cable. It also doesn't show up on desktop or in Disk Utility. Disk drive and cables work fine with my old computer on Snow Leopard. It also wouldn't recognise my old computer via Firewire, when I tried to migrate info across to it (this was before updating system to 10.8.4). However, flash drives show up fine. Any suggestions?

The EHD is a 2.5" WD SATA
Two things:
1) WD drives are fine. It's their enclosures that are useless. I had a desktop WD drive that was nothing but trouble with an older PPC Mac and a newer Intel iMac. Stalled, overheated, and generally made it impossible to use Time Machine.
I ripped the WD bare drive from the enclosure and dropped it in an OWC Mercury Elite Pro and now the WD drive that apparently was the Devil's Own is working flawlessly.
2) Drives that must get all their power from the USB cable can fail to be recognized. As electric motors wear, they can demand more powr. Eventually they need more power than any USB port can provide.
Always get a desktop drive for a desktop computer. Having suffered through serveral "Name-brand" external drives, I now spend the extra money for the OWC products. They are made for Mac and very reliable.
Possible fixes for your old drive:
1) First, make sure the computer is set to "see" the drive. By default, it is not. While in the Finder, open Finder Preferences and make sure "External Disks" is checked:
2) Reformat it as "Macintosh Extended (Journaled)" with Disk Utility. That often wakes up the drive.
3) Get a POWERED USB hub. It makes up the power the drive can't get from teh port.

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