[ubuntu] Partition issue

You chose the full drive LVM with encryption install option.

Standard disk tools do not work on LVM - logical volumes, nor do they see encrypted partitions.
And many have not been housecleaning the /boot partition which then fills up and runs out of space. I think that issue has been fixed with 16.04 as it now keeps last and new keel & deletes all others.

I do not know LVM nor encryption:
LVM - Logical Volume Management.
Advantages/Disadvantages LVM Post #9
http://ubuntuforums.org/showthread.p...45#post9917145
https://wiki.ubuntu.com/Lvm
https://help.ubuntu.com/community/UbuntuDesktopLVM
http://www.tutonics.com/2012/11/ubun...de-part-1.html

/boot full or Not enough disk space on updates
http://ubuntuforums.org/showthread.p...8#post13418808
https://help.ubuntu.com/community/Lu...ng_old_keels
https://help.ubuntu.com/community/Lu...moveOldKeels
Check current keel I also keep one older just in case:
#Current keel:
uname -a

I normally use synaptic to remove older keels:

Determine your current keel:
uname -a
-s is similate so you can review first:
sudo apt-get -s autoremove
sudo apt-get install synaptic
In synaptic search for linux-image to choose to delete old ones
Using synaptic
http://ubuntuforums.org/showthread.php?t=1283521