I'm loosing track of this issue a bit, as I've been dealing with it for
a while. I have a machine that I've setup where /gnu/store is a btrfs
subvolume. I do this so that I can make flexible use of the space on
that btrfs filesystem.

Unfortunately, the grub configuration generated for this doesn't seem to
account for this, and so it requires some tweaking to get it to boot.

A long while back, I discovered I could make the following change, then
the generated grub configuration would be fine.