Q: iMac does not start after last security update

a solution that helped me:

Re: Security Update Causes Apps To Be Damaged

Do you have a Fusion drive. My Mac Mini with a Fusion drive has been rendered unbootable since I applied the update last night.

I have a late 2012 iMac and have the same problem. I had to revert to a backup to the previous software version. I will let you know if I figure this out. I have the 3tb Fusion drive.

Imac 2012 with fusion drive here.... Same problem, a reboot helped but so slow it looks like nothing is responding and the dock never displayed. I used my Superduper backup to restore the backup I took right before the update and all is well now. You need to report this to Apple. I see others with the same issue, mostly iMacs.

I had the same issue and this solved it for me:

1. Startup this Mac with any external MacOS (Yosemite, El Capitan on external disk) or Target Mode.

2. On the problem disk (Macintosh HD) Go to Folder /private/var/folders and trash everything in this folder.

3. Empty the trash and reboot to main MacOS.

I have solved this issue by downloading the file secupd2016-001elcapitan.dmg directly from Apple and running that instead of doing the update through the App Store. The install crashed when using the Update from the App Store and ran perfectly when run from the downloaded file.

I have had this problem before on some updates and the full downloaded update file always solved the problem.

Good to know. I think the App Store update crashed on the install in my case. I'm just real nervous about doing the update from the dmg file though. We'll see.    I might get a case of reckless abandon!