Existing SSD suddenly unrecognised. Internet recovery finds drive.

Hello All

A1278 with 16Gb ram and 250gb WD blue SSD and Sierra. Hardware upgraded to this spec about 18 months ago and was working perfectly. Two days ago the SSD was suddently unrecognised (the flashing folder with a question mark).

Researched on line and swapped the SSD/HDD cable (part number 821-2049-A in my case). The cable didn’t make any difference but I work as a contractor and the client has several A1278 so I borrowed one to check if the new cable was working and it is.

So I found a spare HDD and booted to internet recovery. Disk utility found the HDD but just to be sure I reinstalled the OS (Mavericks in this case). When I rebooted it worked fine. I then put the SSD back in and it didn’t work.

Then I cleaned the contacts at the motherboard end of the above cable.

So then I got a dvd replacement caddy for a different/additional drive. Attached the SSD to the caddy but it would not boot but again internet recovery and disk utility found the SSD. So I again borrowed another A1278 and tested the SSD - all working no issues.

Then I pluged in the HDD into the DVD caddy and booted. It worked fine. But I again reinstalled the OS from internet recovery just to make sure. The reboot during installation worked fine for the HDD in both the original position and in the DVD caddy. This drive is recognised. and mac boots from it in whatever position it is in.

All SSDs and HDDs are all formatted using Journaled. The SSDs are running Sierra.

I have reset the PRAM. When I hold down the option button on boot the mac doesn’t find the SSD but does find the HDD. I have read the article about tape and ensuring a good connection in the SSD end of the cable connecting the SSD/HDD to the motherboard. I have used the tape to no avail and I have also cleaned the SSD contacts with an old toothbrush and Alcohol (Not Vodka but frankly it was vodka, it would be a toss up between using it myself or using it to clean the SSD contacts!).

If I put the SSD in an external caddy and press option no drive is found.

If I try all of the above with a variety of SSDs - all of which work in other machines - they are not recognised unless it’s internet recovery and disk utility.

As 4 non blondes asked..what’s going on? Any and all ideas will be gratefully received.