I have an 2010 iMac running High Sierra. Recently I got a notice that there was an update to my system. I’ve installed a few and they worked fine. I installed this latest one tonight but my iMac will not restart using this new upgrade. When I try to start my computer it loads a small part of the system and then just sits there with no advancing of the slider. I have tried starting in safe mode but no luck. I tried restarting in recovery mode but my recovery disk is apparently not working.

Is there any way to start my computer? I am leaving it in the startup mode all night to see if it will eventually load but I’m not very optimistic. Is there another way to get it to startup? I thought I had a backup disk but that method doesn’t seem to work. I am completely out of ideas and my computer is a brick. Any help would be greatly appreciated.

  • For the record, was it Security Update 2020-004? – lhf Jul 28 at 10:58
  • Have a look at: How to reinstall macOS from macOS Recovery – user3439894 Jul 28 at 14:30
  • @lhf it was a security update but I don't recall the number. It appeared about a week ago. – jmh Jul 28 at 14:38
  • @user3439894, thanks for that link. There is a lot of very useful information in that article and the articles referenced therein. – jmh Jul 29 at 3:13

Start from Recovery & re-install the OS.

This will lay a fresh OS over the old one without affecting anything else on the Mac.

I've had this happen several times on different Macs with High Sierra security updates.

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Well my attempt at installing the System update worked after all. I started it installing and went to bed. When I got up this morning the update had installed and seems to not cause any problems - yet! I think maybe I was too impatient. But when I tried it I waited for over 30 minutes and the slider had not moved. I don't know how long it took but at least I got it installed. So lesson learned is if your update looks like it won't install, let it run all night and see if that works.

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  • Indeed, patience is required. Your 2010 iMac probably has a slow HD. – lhf Jul 30 at 16:36
  • It does indeed. I hoped to buy a new iMac this month when the new ones were hopefully announced at WWDC. Alas, that didn't happen. So i'm still waiting! – jmh Jul 30 at 17:21

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