So I unplugged the SSD and booted from a live USB and since then the SSD is no longer bootable.
I tried re-installing the bootloader as described in this NixOS wiki article.
It didn’t make the disk bootable again.
Any ideas how to fix that?
Are you on a BIOS/legacy or EFI system?
What error do you see when you try to boot with only that SSD attached?
It’s an EFI system.
The problem was that it just would not boot at all, it wasn’t even recognized as a bootable device by the BIOS.
What fixed it was a simple install as documented in the official manual.
But thanks for the reply anyway, cheers.