I've tried to install the 10.9.3 update three times. Each time, the app store downloads, the machine reboots, and then up comes a dialog complaining that it could not verify the update. Upon clicking OK, it reboots and spends 7 minutes putting things back the way it found them.
Disk utility finds nothing to complain of, and there's plenty of space.
Machine is running 10.9.2, and it is a mid-2011 mac mini.
What do I do?