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UPDATE: Got it sorted.
Boot up - when the terminal stops loading the error and gives you the mdadm prompt, type 'blkid'
This should show you all attached disks. You're looking for the one toward the bottom (arrarys/sata disk should be at top). For me it was a bit ambiguous - there were two that my boot USB could have been so I just remembered both (happened to be sdg1 and sdh1).
Reboot, and when you see the bluescrren GRUB hit 'e'
In the above code block change line #15 - so mine became 'root=/dev/sdg1'
Then hit F10 to boot. If it does, you've got the right one - if not, go back and try another disk name/id.
You should not have a 'login' prompt. Login in, then type 'update-grub'. This will make your change to the GRUB permanent.
Most of the above has been said in one form or anything by others, I just had trouble following it as a newb, so hopefully my elongated writeup will help another newb!
This worked perfectly for me. Many thanks.