I realized that the drive had again become disconnected at some point during the process and Testdisk must have continued to probe for the disk without connection. I put the drive back in the housing and ran an MFT repair that began scanning, and after the first evening it had scanned both the MFT and the backup MFT, determining that they were identical and null. This is when I began the Testdisk atttempt in order to recover the file table. I was able to see the entire contents of the drive after I initially removed it from the housing and ran Easeus Partition Recovery, but after recovering a couple of folders about 75% of the folders/files no longer appear under LIST. The bare drive itself is a Seagate Barracuda 4TB drive that came housed as a Seagate Desktop Pro 4TB external drive, PN:1DXAP - 500 4TB.

It seemed to have an intermittent connection when I first started attempting recovery, but now it does not register at all. At first it registered for a few seconds before then disappearing from both explorer and disk management.

I took the drive out of it's housing because the drive does not register at all on my Windows 10 Pro machine when connected via USB between the original housing and the computer.

I'll keep you all updated, thank you for your help so far.Thank you so much for your response. On Saturday, all being well I will have an Ubuntu laptop with an inbuilt reader at my disposal so hopefully I will get somewhere with it all then. However, this kind of not relevant anymore, because unfortunately I borrowed the Pi and the reader together, so I have neither now. Perhaps if the card was detected then /dev/sda or /dev/sdb would become usable, but otherwise are just empty placeholders. On the other hand, as /dev/sda and /dev/sdb appeared regardless of what order i plugged the reader and the SD card in surely they do not refer to the card(s) plugged into the reader? Unless they are 'empty' filesystems or something, waiting to be filled by the card that gets plugged in. And you are correct in thinking that the SD card reader could read multiple types of card. Yes I did try different combinations to try to get the SD card listed in /dev, and plugging in the SD card before / after the reader was one of them.

Crystal testdisk