For the second time in by brief (3 weeks) ownership of my 2015 5k 27" iMac I have had a problem updating OS X. Symptoms are that the update installs until the reboot, I then get a black screen with the cursor and nothing else. I called Apple on the first occasion this occurred and they suggested rebooting in Safe Mode, after which the update continued normally. Trouble is, this has occurred again just now when updating to 10.11.3. Any thoughts on what might be causing this?