Desktops :: External Hard Drive Unable To Mount

Without warning my iMac [OS X Snow Leopard] popped up an alert saying that I should eject a drive before disconnecting it. I had done nothing to generate this error message. I clicked ok to close it and noticed my 4 month old, external 8T Mercury Elite-AL Pro Qx2 was no longer visible in my drive directory. I also noticed this external unit was making a looped grinding noise as if it was trying to write without success. I figured one of the drives failed and took it to the mac doctor. Other than a few directory errors, everything appeared to be fine, so I paid the service fee, brought it home and plugged it in only to find the same prob - obviously it's something with my iMac. Before I go spending more service fees, I wondered if anyone here could point me toward a method of discovering what might be wrong. I ran disk utility [verify and permissions] and fixed some directory errors, but don't know how to look further. iMac was home networked with a PC [Windows] and a laptop [Windows]. I did notice something wacky with the PC, but its very old and I expect it to die any day. I was running the network through the PC, so I direct connected the iMac to the router and modem but no change: still cannot see the external drives. What other info do you need to make an educated guess for me?