Failed: grub-install --target=powerpc-ieee1275

[Peter <petrvz@gmx.net>]

I seem to have a problem with grub being read-only after sudo apt
update & apt upgrade on my iBook G4:

Setting up grub-ieee1275 (2.06-5) ...
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: 270: 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
Setting up linux-image-6.0.0-4-powerpc (6.0.8-1) ...
/etc/kernel/postinst.d/initramfs-tools:
update-initramfs: Generating /boot/initrd.img-6.0.0-4-powerpc
/etc/kernel/postinst.d/zz-update-grub:
/usr/sbin/grub-mkconfig: 270: cannot create /boot/grub/grub.cfg.new:
Read-only file system
run-parts: /etc/kernel/postinst.d/zz-update-grub exited with return
code 2
dpkg: error processing package linux-image-6.0.0-4-powerpc (--
configure):
 installed linux-image-6.0.0-4-powerpc package post-installation script
subprocess returned error exit status 1
dpkg: dependency problems prevent configuration of linux-image-powerpc:
 linux-image-powerpc depends on linux-image-6.0.0-4-powerpc (= 6.0.8-
1); however:
  Package linux-image-6.0.0-4-powerpc is not configured yet.

dpkg: error processing package linux-image-powerpc (--configure):
 dependency problems - leaving unconfigured
Setting up linux-image-5.19.0-1-powerpc (5.19.6-1) ...
/etc/kernel/postinst.d/initramfs-tools:
update-initramfs: Generating /boot/initrd.img-5.19.0-1-powerpc
/etc/kernel/postinst.d/zz-update-grub:
/usr/sbin/grub-mkconfig: 270: cannot create /boot/grub/grub.cfg.new:
Read-only file system
run-parts: /etc/kernel/postinst.d/zz-update-grub exited with return
code 2
dpkg: error processing package linux-image-5.19.0-1-powerpc (--
configure):
 installed linux-image-5.19.0-1-powerpc package post-installation
script subprocess returned error exit status 1
Errors were encountered while processing:
 grub-ieee1275
 linux-image-6.0.0-4-powerpc
 linux-image-powerpc
 linux-image-5.19.0-1-powerpc
E: Sub-process /usr/bin/dpkg returned an error code (1)