I meant to add that the problem of old kernels filling the /boot partition 
is a known issue:

http://ubuntuforums.org/showthread.php?t=2238152

With Ubuntu 13.10 and later, we can use this command...

sudo apt-get autoremove

...which (the page says) "will now removes those older kernels (images and 
header and related files too), leaving the current kernel and also 1 
latter kernel."

Seems reasonable.

Mike



On Wed, 2 Sep 2015, Mike Miller wrote:

> Does this sound like a reasonable plan?  After backing up all files and 
> booting to a Live CD:
>
> (1) mount /dev/sda and /dev/sdb
> (2) Remove /boot directory from / partition on both sda and sdb
> (3) Install grub in /boot parttions on both drives.
> (4) mount /boot partitons of both sda and sdb
> (5) copy known-working kernel and two newest installed kernels to /boot 
> partition from backup media (formerly in /boot of / partition)
> (6) make sure /boot partition is mounted in fstab
> (7) /boot partition should also be in the RAID1
>
> Does that sound like it would fix it?  Any ideas?
>
> More info below, if you're interested.  ;-)