Hello.
After recent minor package upgrade (I'm on sid) I found that the root
filesystem is in 'RO' mode. I changed 'ro' into 'rw' in grub.conf and
after reboot everything works fine. Any update which triggers
update-grub causes 'ro' back.
I tried to find what's broke, but failed.
I can't find who remounts root filesystem into rw mode. Is this some
systemd generators magic? Or is this happens inside initrd?
As far as I know, it's mounted ro, so that initrd can check if it is okay (via fsck). And initrd is then supposed to remount it rw.
If it can't be automatically verified, you get dumped, unceremoniously into a busybox prompt *inside* initrd. (I know, because I troubleshooted a friend's Buster laptop, after he pressed the power button, during an apt upgrade. Ugh!)
Good luck!
Kenneth Parker