Note to self: the next time GRUB starts exiting with 1 during
postinst (when upgrading the kernel), check
/boot/grub/device.map. Apparently the last time this happened, I had to change
hd0 to point to
sda, and this time I had to change it to point back to
hda. Without this change
grub-probe would fail, causing
update-grub and thus the
postinst to fail also.
Also, don’t be fooled by
update-grub staying quiet;
update-grub && echo success lets you know whether it did actually complete or not.