...when you upgrade your Mac OS to forget to temporarily disable the overnight backup to an external drive.
Trying to eliminate specifics as to why I can't play that RAF Tornado video properly on my iMac, I decided to migrate from Mavericks (10.9) to El Crappo (10.11) which is as far as I can go with my geriatric iMac. I made a backup copy of my drive so that if it all went pear-shaped I could go back to Mavericks and iPhoto (as I hate Photos). I also hate the childish layout and font style of El Crappo and also the childish layout in Mail....yet more bloody wasted white space.
So I installed El Crappo late one evening, seemed to go OK...left it to cook overnight. Came down in the morning and saw the overnight message from my backup program....the automatic schedule had overwritten my Mavericks backup and so I was stuck with El Crappo.
I then noticed that App Store told me I had some security updates needed but the download seemed to have stuck. So I restarted only to be presented with a grey screen and the logo that says 'You are stuffed, mate, as I can't boot up this OS'. And the backup had the same dodgy version of the OS.
Rather surprisingly I didn't go into a paddy and hurl my iMac into the corner. Sang-froid is my middle name. I guess what with everything that's gone on (and often wrong) with the house that all my angst has been used up...to be replaced by a feeling of numbness.
Apple leapt to the rescue as the emergency restore s/w on the drive let me downloaded a fresh copy of El Crappo from Apples' website. Three hours later and we're back up and running with an updated copy of El Crappo. But I really find the user interface childish and loathe Photos. But Hey Ho...at least it's working.
Mind you not too sure whether my photos are all intact. There were some hidden ones that I can't find. Seems to have duplicated a lot as well.