"cannot create /boot/grub/grub.cfg.new: Read-only file system" on a Power Mac G5

apt-listchanges: Reading changelogs...
(Reading database ... 131680 files and directories currently installed.)
Removing linux-image-6.1.0-9-powerpc64 (6.1.27-1) ...
/etc/kernel/postrm.d/initramfs-tools:
update-initramfs: Deleting /boot/initrd.img-6.1.0-9-powerpc64
/etc/kernel/postrm.d/zz-update-grub:
/usr/sbin/grub-mkconfig: 270: cannot create /boot/grub/grub.cfg.new: Read-only file system
run-parts: /etc/kernel/postrm.d/zz-update-grub exited with return code 2
dpkg: error processing package linux-image-6.1.0-9-powerpc64 (--remove):

installed linux-image-6.1.0-9-powerpc64 package post-removal script subprocess returned error exit status 1
dpkg: too many errors, stopping
Errors were encountered while processing:
linux-image-6.1.0-9-powerpc64
Processing was halted because there were too many errors.
Error: Sub-process /usr/bin/dpkg returned an error code (1)

root@debian:~# apt-get purge linux-image-6.1.0-9
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Note, selecting 'linux-image-6.1.0-9-powerpc64' for regex 'linux-image-6.1.0-9'
The following packages will be REMOVED:
linux-image-6.1.0-9-powerpc64
0 upgraded, 0 newly installed, 1 to remove and 991 not upgraded.
3 not fully installed or removed.
After this operation, 302 MB disk space will be freed.
Do you want to continue? [Y/n] Y
(Reading database ... 131680 files and directories currently installed.)
Removing linux-image-6.1.0-9-powerpc64 (6.1.27-1) ...
/etc/kernel/postrm.d/initramfs-tools:
update-initramfs: Deleting /boot/initrd.img-6.1.0-9-powerpc64
/etc/kernel/postrm.d/zz-update-grub:
/usr/sbin/grub-mkconfig: 270: cannot create /boot/grub/grub.cfg.new: Read-only file system
run-parts: /etc/kernel/postrm.d/zz-update-grub exited with return code 2
dpkg: error processing package linux-image-6.1.0-9-powerpc64 (--remove):
installed linux-image-6.1.0-9-powerpc64 package post-removal script subprocess returned error exit status 1
dpkg: too many errors, stopping
Errors were encountered while processing:
linux-image-6.1.0-9-powerpc64
Processing was halted because there were too many errors.
E: Sub-process /usr/bin/dpkg returned an error code (1)

root@debian:~# cat /etc/fstab
# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# systemd generates mount units based on this file, see systemd.mount(5).
# Please run 'systemctl daemon-reload' after making changes here.
#
# <file system> <mount point>   <type> <options>       <dump> <pass>
# / was on /dev/sda3 during installation
UUID=78d8bc92-2c65-4282-86b6-e305a3b8a1e3 /               ext4    errors=remount-ro 0       1
# /boot/grub was on /dev/sda2 during installation
UUID=32bf2d07-b070-372c-b1a5-5e304b1597cb /boot/grub      hfs     defaults        0       2
# swap was on /dev/sda4 during installation
UUID=f59b067e-06eb-40b0-b45b-0928f1945cda none            swap    sw              0       0
/dev/sr0        /media/cdrom0   udf,iso9660 user,noauto     0       0
root@debian:~#
root@debian:~#
root@debian:~# blkid
/dev/sda4: UUID="f59b067e-06eb-40b0-b45b-0928f1945cda" TYPE="swap" PARTLABEL="swap"
/dev/sda2: UUID="32bf2d07-b070-372c-b1a5-5e304b1597cb" LABEL="untitled" TYPE="hfs" PARTLABEL="untitled"
/dev/sda5: PARTLABEL="Extra"
/dev/sda3: UUID="78d8bc92-2c65-4282-86b6-e305a3b8a1e3" BLOCK_SIZE="4096" TYPE="ext4" PARTLABEL="untitled"
/dev/sda1: PARTLABEL="Apple"