Update: it worked without any issues after i tried a different USB stick with a different ISO. Which is weird, because I had installed LMDE on several machines with exactly this stick. I guess the ways of our Lord and Saviour are mysterious.
I swapped out the SATA drive on the Dell 5070. There is no NVME drive. Before I had a 256 GB drive. I put in a 1 TB drive and installed Mint on it. The previous drive also had Mint. But I must have somehow messed up the BIOS settings because now the blasted thing won't boot.
The drive shows up on the System Info page of the BIOS:
It also shows up in the Drives page:
But I can't choose it as a boot option:
Clicking on Add Boot Option only brings an error that it can't find any file system. I tried restoring the settings but that didn't help. What can I do?
Just had fun with this with my optiplex 790.
Things I have found:
If you are using the front USB ports try the lower ports.
Make sure that you have formatted your live USB properly.
On boot press F12 to get the one time boot menu, if everything is right you should get a menu that gives you legacy boot options with UEFI boot options below that.
The big thing here is that not all live distros appear to work with the Dell UEFI implementation (got Linux Lite and Manjaro working)