After dismissing the notifications for months I finally let macOS upgrade from Mojave to something more recent, I think Catalina. The process failed, and I get a black screen at startup. After trying many other things I bought a new hard drive and restored from time machine on there. I can boot using this external drive and I now simply want to get the stuff that wasn’t backed up. But whenever I mount the internal disk, the machine restarts. How can I prevent this?