Originally posted by jonnin:

the simple way to do it is to use a symbolic link. If you do that, the game writes where it thinks the old folder was, and your link redirects it to the new one.
this requires moving (I recommend this) the old folder to the new location with all your files and putting the link where the old one was. Back it all up before you try anything, but a simple test would be to make a new text file in like notepad and save it to the valheim folder and watch it appear in the other place instead.

if the symbolic link on the dying drive also craps out, its not going to work. But those are like 50 byte files, and usually don't fail assuming the drive is going to keep working well enough to do anything at all with. Do a scandisk and lock/mark the bad sectors so it stops trying to use them every so often helps reliability too, as does keeping the drive at least 10% or so free space.

this is way too many words for it, but its what came up and I don't want to write it all out if you don't need me to.
https://www.howtogeek.com/16226/complete-guide-to-symbolic-links-symlinks-on-windows-or-linux/

if you do get stuck, I can figure it out for you tonight or tomorrow.