Looks like it was my hard drive that was causing the issue.
Booted up again and fsck reported a problem in /
Clicked M as instructed to go into Manual mode and then ran fsck and all subsequent fixes.
Rebooted.
Then went into Update Manager and was prompted to do a partial upgrade. This completed successfully and after another reboot, UM now reports my system as up to date.
Looks like it was my hard drive that was causing the issue.
Booted up again and fsck reported a problem in /
Clicked M as instructed to go into Manual mode and then ran fsck and all subsequent fixes.
Rebooted.
Then went into Update Manager and was prompted to do a partial upgrade. This completed successfully and after another reboot, UM now reports my system as up to date.