"I am trying to recover a iMac HD.
...
It seems to be running (the HD), but it won't be recognized on my computer"
I assume that the drive in question has been removed from the iMac and put into an external case, is this correct? You're not very clear in your original post.
Diagnostics can be difficult when you've got the drive "entombed" in a case like that.
Is the drive in question ATA/IDE, or is it SATA?
If it's a SATA drive, I think you'd do better to get one of these:
http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias=aps&field-keywords=usb+sata+dock&x=0&y=0
... and then try to do the diagnostics and recovery on the "bare" drive in the dock.
Doing it this way makes it easy to separate "drive problems" from "enclosure problems".
As Miker2k said, the first thing to determine is if the drive is physically "spinning up" when you connect it (again, the "dock" makes such things easy).
Of course, if it doesn't spin -- dead drive.
If it does spin, next use Disk Utility to check if DU can "see the presence" of the drive on the USB bus. Does it?
If the drive is "seen", but won't mount on the desktop, could be directory problems.
DU can fix _some_ directory problems.
If DU can't fix it, there is DiskWarrior, which can repair directory problems that DU can't.
But it may go beyond that, to where you have to give up on trying to "repair" the drive's directory, and look instead to "data recovery" ("recovery" as distinguished from "repair") applications.
Of these, Data Rescue and Stellar Phoenix Data Recovery are two with good reputations.
However, there is a newcomer to the data recovery scene and I would suggest you give it a try first:
http://www.cleverfiles.com/
For a time, the program is still "in beta" -- but if it works, there is no "upcharge" (as you have to pay for recovery with DR and SPDR).
It's worth a try....
