After doing this...:
sudo apt-get update sudo apt-get upgrade
... I found the terminal hanging.
ps reports this:
17687 pts/2 D+ 0:00 /usr/sbin/grub-probe --target=device /
This is on an Amazon EC2 server, which is otherwise performing its duties, so I could ignore this issue, except it stops me doing any further upgrades:
E: Could not get lock /var/lib/dpkg/lock - open (11: Resource temporarily unavailable) E: Unable to lock the administration directory (/var/lib/dpkg/), is another process using it?
How can I recover from this?