Yesterday I tried to upgrade my fathers 2012 iMac from El Capitan to Sierra. Unfortunately after 30 minutes the process stopped with an “internal error”. I searched the internet what to do about this. I got a lot of tips like reset PRAM/NVRAM, boot in safe mode etc.. The safe mode didn’t work however. Every time I held shift when rebooting, some characters appeared on the screen and the iMac booted again immediately. I did a PRAM/NVRAM reset, but had some doubts: what if we waited again 30 minutes and that “internal error” popped up again? It would be more trial and error this way. At the end I found that we wasted too much time. I rebooted the iMac in Recovery mode (which worked thankfully) and restored from a Time Machine backup. I had already read about some people having problems installing the first release of Sierra but had hoped that this would be solved now. My question is: what can I do and/or what tips do you have so I have the biggest chance of successful upgrading to Sierra? I want to make sure we won’t spend time with trial and error and waiting 30 minutes every try. Thanks in advance.