I tried an apt upgrade and it tried to upgrade grub-ieee1275 on a G5 iMac (ppc64) which failed.
Setting up grub-ieee1275 (2.12~rc1-12) …
Installing for powerpc-ieee1275 platform.
grub-install: error: cannot backup `/boot/grub/powerpc-ieee1275/adler32.mod’: Read-only file system.
Failed: grub-install –target=powerpc-ieee1275
WARNING: Bootloader is not properly installed, system may not be bootable
/usr/sbin/grub-mkconfig: 274: cannot create /boot/grub/grub.cfg.new: Read-only file system
dpkg: error processing package grub-ieee1275 (–configure):
installed grub-ieee1275 package post-installation script subprocess returned error exit status 2
Also during boot I see an error.
Nov 14 12:34:33 debian systemd[1]: Mounting boot-grub.mount – /boot/grub…
Nov 14 12:34:34 debian systemd[1]: Mounted boot-grub.mount – /boot/grub.
Nov 14 12:34:38 debian systemd[1]: Starting grub-common.service – Record successful boot for GRUB…
Nov 14 12:34:39 debian grub-editenv[503]: grub-editenv: error: cannot open `/boot/grub/grubenv’: Read-only file system.
Nov 14 12:34:39 debian systemd[1]: grub-common.service: Main process exited, code=exited, status=1/FAILURE
Nov 14 12:34:39 debian systemd[1]: grub-common.service: Failed with result ‘exit-code’.
To solve this problem I had to do a filesystem check in the boot partition:
fsck.hfs /dev/sda2