GETTER computer, my 2016 build with four AMD compute cores, the household workhorse Linux computer, collapsed in October 2017. Ubuntu 16.04 LTS (long term support) sda (first SATA drive) filled up the /boot partition (due to Ubuntu automatic updates downloading through Internet) and lost ability to get beyond user login, Oct 12.
Spare room on the /boot drive had periodically gotten low, and I learned to go in and remove the oldest set of four files like abi, config, System.map, and vmlinuz, which came about once a month and amounted to 5MB per set, but the more mysterious initrd.img, a RAM drive archive, was the bigger offender at 44MB and I never learned if it was safe to remove the older ones. For two weeks, the daily warning about zero room left in /boot appeared on the screen, and I just tried ignoring it, and the computer kept working.
But one day, after user login, it flashed a screen completely full of static, random, colored pixels. This went away after 10s, revealing the customary desktop, but only the color background without any icons or menus. HDD lamp flashed on tower. I used the "live" .iso USB memory to boot, and this indicated the computer hardware was OK. Using the live USB try-it-before-you-install, or something similar, I was able to prune 4 months of the set-of-four smaller files out of /boot but that didn't bring it back.
This computer had five HDD all along, to facilitate mdadm software RAID, but I hadn't done mdadm nor mounted any more partitions using the DISKS application. Used another of the five HDD, a second WDC RE4 drive, as a new sda to install Ubuntu 17.04 after disconnecting the 16.04 sda. Upon reconnecting the 16.04 HDD, it is sdb (no surprise) and the LVM volume is sdb3, the third partition on that HDD. Tried the mount command with guidance from Internet blog, but I had checked the box for LVM when I had installed 16.04 a year and a half ago. LVM is not a simple partition and you don't just mount a logical volume. After accumulating maybe 8 hours of work on this problem, I came across a web site that said to get-apt install lvm2 to enable use of vgscan, vgs, lvdisplay, and most importantly vgchange -ay //Activate all volume groups available. Suddenly in the left-edge launch strip, a new HDD icon pops up. It is the LVM partition and I can read files in it! I got 50GB copied over into 17.04's Documents. This rescued the data, but I will have to reinstall programs. I went ahead and partitioned one of the Seagate HDD for data backup. It is showing up on reboots without having an entry in /etc/fstab, the file-system table; I had mounted it through the DISKS application by clicking on something, maybe that is enough.
As 17.04 installed, I did not check the LVM check box! I need to find out if those 44MB initrd.img archives in /boot can be deleted. But here is the funny thing -- /boot is a folder, not a partition. Why did it fill up? There is plenty of room free in the partition. During previous Ubuntu installs, I have always wanted to manually partition a bigger /boot partition, and some other Linux partitions, upon installation, but with EFI and GUID coming in, I don't know enough about partitioning.