SSD has disappeared from Disc Utility only OS X Base system

What version of macOS installer were you booting when booting into Recovery Mode? Sometimes a Mac will not boot into the proper online macOS installer and instead boots to the oldest online macOS installer. If you have a Retina model laptop with a third party SSD installed, then you need to be booting to the macOS 10.13+ installer. Older versions of macOS will not be able to see a third party NVMe SSD since older versions of macOS lack the necessary NVMe SSD driver.

If you are booting to macOS 10.13+, then within Disk Utility click "View" and select "Show All Devices" so that the physical drives appear on the left pane of Disk Utility. If you do this with macOS 10.13+ and you don't see the internal SSD, then either the SSD, SSD adapter, or the Logic Board is bad.

mqsasia wrote:

There's good news, and bad news, swapped both and the laptop is all singing and dancing with the original 256Gb SSD. The now external Transcend seem to have died, although the blue light is burning it does not show up in finder or DU, less then a year old and not cheap, wonder how is that possible as these are supposed to be a decent qual, any ideas? Could it possibly be repaired?

As I thought this is an NVMe based SSD which requires macOS 10.13+. Your pictures showing Disk Utility are older versions from macOS 10.12 or earlier which is why the Transcend SSD isn't showing up in Internet Recovery Mode. If booting into Internet Recovery Mode using Command + Option + R only boots to an old version of macOS installer, then create a bootable macOS 11.4 Big Sur USB installer now that you can boot your Mac with the original Apple SSD.

How to create a bootable installer for macOS - Apple Support

I honestly don't know if Yosemite's Disk Utility will be able to see an external NVMe SSD especially since the drive would be using the APFS file system which Yosemite does not understand. In theory Yosemite should at least see the physical SSD, but sometimes macOS won't even see the physical drive if something on it is unexpected or unknown. It doesn't always mean the drive is bad. It is best to try using macOS 10.13+ to be sure.

Question1. ;

Reinstall Big Sur (curr Yos) and should I then be able to use my Time Machine external to reinstall a back up right, what are the steps to reinstall a back up as haven't done this?

I don't use Time Machine, but I believe you will want to perform a clean install of macOS first. After a clean install, then you can restore from the TM backup when you first boot into the newly installed OS (there will be an option to do this).

Before you go this route I suggest creating a bootable macOS 11.4 Big Sur USB installer. Swap SSDs again and boot to the Big Sur USB installer to see if you can now see the Transcend SSD with Disk Utility. You will need to click "View" within Disk Utility and select "Show All Devices" before the physical drive and hidden Container appears on the left pane of Disk Utility. Try running First Aid on the physical SSD, then run First Aid on the Container. Even if First Aid shows the "Container" is "Ok" click "Show Details" and manually scroll through the report to see if there are any unfixed errors listed. If there are any unfixed errors, then you will need to erase the whole physical SSD before restoring from a backup or clone.

I recommended swapping SSDs for the above procedure so that you don't accidentally mess up your bootable Yosemite on the original Apple SSD, but you can also do this with the Transcend SSD installed in the external enclosure. Sometimes macOS touches things you don't expect.

You are booted to an older version of macOS as shown by the old version of Disk Utility in your photos. If the Transcend SSD is an NVMe SSD, then it is no surprise Disk Utility doesn't see the internal SSD. If the Transcend SSD is a SATA based PCIe SSD, then it should be seen by Disk Utility.

Let us know how the SSD swap goes.

It's either not properly connected to the computer or dead. Fixing either of these requires a hardware repair, and data on a failed drive can only be recovered by a professional service.

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