08-24-2018, 01:23 PM
I encountered this same problem last night when I installed the 18.04 release upgrade. Several attempts to extend the /boot partition failed miserably. Then it finally dawned on me that I already had a copy of /boot on my rootfs partition, so I just did a "umount /boot", and then ran the update. This time, the updater saw the "/boot" on rootfs, where there was plenty of room, and the upgrade continued just fine.