Don't you have three of them...the one you had at the start of this thread, the one you bought on eBay, and the one you "found?"
I ask because you've only reported having tried two with the iMac, and if the original and eBay ones worked but not the found one it would suggest that the found one is defective...not that that contributes anything towards solving your problem.
In other words: Basically, I'm stumped!
