SATA Controller Failure or SATA Cable Failure?

Hello! I have been tearing my hair out over this repair, so I’d be grateful for any advice you can give.

My Mum’s iMac was running poorly for several months - it took ages to boot, and would hang on the spinning wheel between every operation. Eventually, it refused to boot (kernel panic I think). I booted into recovery mode, and was unable to repair the drive, either through disk utility or command line. I figured we had a bad drive and decided to replace it and upgrade to an SSD at the same time.

I bought the repair kit, including the sensor cable and a 3.5-2.5” bay adapter. Important detail - I formatted the new 960gb SSD with my MacBook Pro via a USB adapter. I then installed it into the iMac, put the machine back together and booted it into the Mojave installer via USB. Mojave installed onto the single partition I had created from the MacBook, and it ran perfectly. I figured everything was sorted, so I left the iMac on overnight to transfer some files (which I had recovered from the old drive).

The next morning, I came down to find the iMac running on a black screen. I can only assumed it had restarted to install an update. I forced it to reboot, and I was presented with the ‘prohibited’ symbol. I booted into internet recovery mode and tried to repair the drive, which failed. I then tried to erase the drive, which also failed. I tried to partition the drive - that failed too.

Next, I tried booting into TechTool’s diagnostic software, which told me the disk passed SMART testing, but did nothing else to fix it.

Next, I reinstalled Mojave onto an external drive, booted from that, and tried to erase/partition the internal SSD through disk utility. That failed every time too. I then ran DiskWarrior, which told me the disk was badly corrupted and couldn’t be fixed.

This surprised me, as it was a brand new disk which appeared to work fine. So I gritted my teeth and cut the iMac open again.

I hooked the internal SSD back up to my MacBook again using a USB adapter which was itself connected to the sensor cable attached to the SSD. I opened disk utility, and, to my amazement, I was able to perform all operations on the SSD - erase, partition etc.

This led me to believe that the problem does not lie with the SSD or the sensor cable, but with either the SATA cable or the SATA controller in the iMac. To test this, I connected the SSD back to the iMac, reconnected the screen (no adhesive), and booted into a Mojave installer USB. As when I had first installed it, the computer recognised the SSD and the ‘Macintosh HD’ Volume I had just created from my MacBook. I tried erasing that volume from the iMac and it once again failed in precisely the same ways. I reconnected the drive to my MacBook via USB, and it erased with no issues.

Therefore, it seems reasonable to conclude that there is something unique to this iMac which makes it unable to erase/partition this drive. I think that this can only be either:

• The SATA cable (defective or poorly seated), or
• The SATA controller.

But I don’t know which! Can anyone advise on what the problem is most likely to be?

Thanks in advance!

Alex