El Capitan 10.11.5

I was at a gig last night where the DJ I was supposed to replace (it was warm-up for a concert, so not a very high stress “gig”) was using the slow time to update his Mac. The little progress bar got stuck at about 85% and never moved again. We did the boot to safe mode thingie, which worked. However, rebooting the Mac after that had the progress bar back and hanging again. He was playing off of RB sticks, so not having the Mac working was no biggie for that particular moment, but he needs the Mac for a gig on friday. Needles to say he was not a happy camper, even in the knowledge that he has a spare Mac just in case (which hadn’t be synced with his main Mac in quite a few weeks though).

So, I’d make sure I have a solid backup (TimeMachine) before starting the process and only starting the process when you know you have plenty of time to deal with any adverse consequences.

I’ll probably get to it this weekend.

Honestly I NEVER had such a problem. You have to be a total idiot to update to a new OS if your work depends on it as soon as it comes out.
El Capitan was the same, not a single problem. I waited until the Serato webpage said “supported”, waited another 2 weeks to see if early adopters are sending in any fault reports and then upgraded. Working ever since.
Also never had a problem with minor upgrades like 10.11.5. Works flawlessly (with latest Serato version).

The upgrade bar of death is a common problem with some MacBooks and there is a workaround and people even got their books replaced. But you need to check the apple website with your serial if you are eligable.

In my book all DJs that have problems like that expect too much and put too little effort into maintaining their gear.
Like you did the right thing, if you do not know where to look, ask at a friendly place (like here hehe) and you will be helped. I even post most of the time when stuff is working with new OS versions.

And as always: Always have a backup before upgrading.

All of these suggestions apply for Apple AND Windows computers.