If it crashes, or you pull the plug, the /boot filesystem may not be mounted cleanly as rw. Run mount and check the mount options, if it is showing ‘ro’ then you will need to unmount the partition, run ’ sudo fsck.hfsplus /dev/sda1’ (assuming /dev/sda1 is your /boot partition) and remount it ‘sudo mount -o force,rw /dev/sda1 /boot’. Otherwise, any OS updates will not work, because the new kernel image will not be saved on /boot.
HTH.
PS. Oh! I just saw this sticky post - more detailed and eloquent than mine above. 