Just like it did when I upgraded from OS X Mavericks to OS X Yosemite a year ago, attempting an upgrade from OSX Yosemite to OSX El Capitan hung again in the same nondescript manner – a blank/empty screen with the pointer, for seemingly infinite period.
I tried to wait it out for a few hours (surely a 30% filled SSD shouldn’t make it need longer?), but that made no difference. The good bit is that this state is easy to recover from – just hold your power button down until the machine powers off, and then restart it (it will go back into OSX Yosemite or whatever old OS state you had running, without issues)
I ended up recalling the fix I did the last time, i.e. moving
/usr/local out of the way, given I’m a heavy Homebrew user, and gave it a try again and the upgrade succeeded in under 20 minutes in the subsequent attempt.
Before the upgrade, I ran in the
sudo mv /usr/local ~/local
And after the upgrade (with a
brew update command just to be sure it is not broken after the move-back):
sudo rmdir /usr/local && sudo mv ~/local /usr/ brew update