That is simple. About as simple as it gets. The more complex method involves figuring out what VPN software Mullvad really uses, figuring out your keying material, fighting with NetworkManager…
Not being able to select boot order in BIOS suggests something very strange is going on, because it suggests that the BIOS can’t see all the drives. That has to happen before the bootloader can be evoked.
It sounds like GRUB is installed on the WD Black. BIOS -> drives it can see -> boot loader
What was the specific error that the Arch boot attempt threw? How did os-prober work for you?
Neither do I. I’ve had a sensor net watching for Wayland news (because sooner or later I’m going to have to migrate to it, just want to know when) but so far there hasn’t been any executive summary.
Your chances are pretty good if you copy them back - ultimately, that’s what the restoration function of backup software does.
As for ownership of the directories and files, that’s a bit trickier and might involve some trial and error. root:root is a safe bet for most of it, but there is a lot of stuff in /var that is owned by system accounts.
What distro are you running? That’ll help figure it out.