Getting this error while downloading grub.
ubuntu@ubuntu:/boot/grub$ sudo grub-install /dev/sda
Installing for i386-pc platform.
grub-install: error: failed to get canonical path of `/cow'.
If you don't want to risk unmounting /cow, I found a neat little trick which fools grub-probe into thinking there is an empty filesystem on /cow and then ignores it.
dd if=/dev/zero bs=1M count=1 of=cowfile
mkfs -t vfat cowfile
mv cowfile /cow
This worked for me running on a live usb wher grub-mkconfig wasn't working.