I had a call from our local Apple Care service center who currently have one of our 27" iMacs in their workshop. The Wifi card needs replacing and the HDD is a 3TB drive which is part of a replacement program. We have firmware passwords on all our Macs (which has been removed from this iMac) and we control Wifi using Managed preferences. We also recreated the recovery partition using Recovery Partition Creator and the Macs are all running Mavericks. We have not enabled FileVault.
So there are 3 problems.
You could possibly blame 2 and 3 on a dodgy logic board (?) but that doesn't explain why the internal HDD needs to be in the old iMac and won't boot or be mounted on anything else. Does anyone have any idea because our Apple Service Centre isn't getting anywhere with conversation with Apple - they are stumped as well. The 3TB hadn't actually failed - it is just part of a replacement program so they want to replace it while it is in the workshop.
Thanks @pblake Originally the Mac was delivered to the service centre with the firmware password active. It was then removed and the drive was taken out afterwards. I could ask them to reconnect the drive and activate then deactivate the firmware password and see what happens.
Still can't think of a reason why the HDD wouldn't mount in any other Mac even if the firmware password was active on the original Mac. Something is wrong there