17.04 hangs during boot at file system mount

Problem: 17.04 hangs during boot at file system mount. It gives a message that cycles between attempting to mount the various systems, then after 1 min 30sec times out, and drops to recovery shell. Blkid shows that all file systems have been detected. The only file system that is mounted is the root system, and that is mounted ro. If I try to mount manually, for example using "mount /dev/sda2", the mount command appears to execute with no errors, but the file system does not mount.

I also have 16.04 and debian jessie, and both boot and mount all file systems with no errors or problems.

This problem first appeared in 16.10, but I no longer have that on my system to test. It has gotten much worse in 17.04.

The 17.04 system was upgraded from 16.04 -> 16.10 -> 17.04. I do not have a spare partition to wipe for a "clean" install.

I have removed/purged the nvidia drivers, am using nouveau.

These file systems are ext4, no RAID, no uefi bios. Dell XPS630 amd64 system. All file systems have been fsck'd and are OK.

Strange thing is that sometimes 17.04 does boot, but most of the time it does not, hanging at the file system mount stage. Not sure what makes the difference.