iMac, "cannot mount root error" occurs during safe boot

During a "safe boot" I get the error "cannot mount root error" occurs. The iMac will not boot under any HDD including recoveries. Tried replacing HDD but get the same result. I normally get the disallow icon. But sometime I get the grey pa with a progress. It starts fast and gets slower and slower until it stops and sits there. Once I left it there for a couple of hours. Having already got the iMac apart i took the display off and the platter HDD was really HOT.

Background - This is a Mid 2009 24" iMac that is used as a home iTunes server. It was using it with one of our Apple TV's, paused it, and when I came back it had lost its connection with the iMac. So I checked iMac and it was running, Mouse woke the display but nothing else would happen. So I rebooted and that was that.

Just to add that it runs a home setup fusion drive consisting of 1 tb platter drive that came with it (think that size is right) fused with a 60 gb SSD. My first thought was it was either a failed group or one of the drives had failed. But with a new blank apple formatted drive I was still unable to boot into recovery. I noticed that the entire time I have never seen the network menu appear. I tried all the boot commands I can find. I can't get access to an online recovery or a hardware check etc. I have refreshed the NVRAM a few times with no change. Removed 1 RAM chip and refreshed again.

I can find plenty of people asking this question but no answers. Best I get are suggesting starting from recovery or something etc. which just won't happen. I can't find anyway to replace the EFI. I'm assuming either a component has failed or the EFI has corrupted.