Preliminary notes. I wasn't exactly sure what I was doing here. Maybe it was already mounted and that was the problem. Anyway, files saved.
A USB boot disk with luks2 and LVM refused to boot. Needed to save the files from it.
I connected it to another computer.
Used this to figure out its /dev/sd(letter)(number):
sudo lsblk -o NAME,FSTYPE,SIZE,MOUNTPOINT,LABEL
It was /dev/sdc6. Opened it into a logical volume name that I made up, "my_encrypted_volume":
sudo cryptsetup luksOpen /dev/sdc6 my_encrypted_volume
Made a local mount point in the home directory
mkdir -p mnt/rescue
Mounted it there:
sudo mount /dev/mapper/my_encrypted_volume mnt/rescue
And that did not work for me, it may work for you. I got an error message about:
mount: unknown filesystem type 'LVM2_member'
I figured out its LVM(?) name by issuing:
Then took that name, e.g.: "root-vg" and issued:
sudo vgchange -a n root-vg sudo vgchange -a y root-vg
…to make Linux mark it as inactive, then active again. Got this info from this page:
And then it could be mounted and then accessed under $HOME/mnt/rescue. Copied files off of it with:
rsync -rh --progress mnt/rescue/path/to/files /media/username/mybackudisk/backups/
The lsblk dance I got from https://askubuntu.com/questions/182446/how-do-i-view-all-available-hdds-partitions .