Hardly had I said that MBR had caused me no problems in years, guess what? It
did. I tried to mount /boot (ext2) to install a new kernel but it couldn't be
mounted - wrong fs type or some such. When I got it mounted as ext4 it looked
fine. Fsck had found nothing wrong either, but fdisk showed the wrong partition
as Active. /boot is on /dev/md1 which is RAID-1 built on /dev/sd[ab]1, so sda1
should have been marked Active (it was when I set it up), but the spare, empty
sda2 was instead.
On booting a rescue system and correcting the Active flag with fdisk, all the
files in it disappeared. So I put them back in and booted the main system. Same
all over again.
In the end I just zapped /, /var, /usr/portage etc, restored from an older
backup, synced and emerged -e world. Then restoring the files in /boot resulted
in a stable system.
Looks like time I investigated GPT or BTRFS. But I don't want to go from GRUB
legacy to GRUB-2. Is that possible?