Unable to start up after security update 2017 001

I see. The installer below doesn't seem to have been changed since first released : it has a modified date of 26 October and the files within show dates earlier than that, as expected.

Security Update 2017-001 macOS Sierra

I wouldn't let that stop me trying again, despite a previous problem & reports on the internet.

Keep current backups, especially before installing software or OS updates.

If you have the same problem again : try starting in Safe Mode at least, before restoring a backup or reinstalling macOS.

I had no difficulties with 2017-001 a couple of weeks ago, although every update finds a number of people having problems. Did the internet give any more details about exactly what needed addressing ?. I don't believe there has been any change to the installer.

Occasionally, although not so much with security updates; existing 3rd party software can cause trouble with restarting after an upgrade or installation. Many people here have used Etrecheck to readily show such things.

I have a 2010 MacBook Air, and it was running 10.12.6, with a 256GB SSD, of which 100 GB was free. With this update, upon restarting, it proceeded to boot until the progress line showed 100% complete and it just stayed there, never finishing the start up.

After trying Safe Boot (worked but still would not boot), booting into Recovery and checking the disk with Disk Utility (it was ok), resetting PRAM and NVRAM (still did not boot), and reinstalling the OS from the Recovery disk (still would not boot), I decided to completely erase the drive and do a clean install, feeling that this was safe since I had two recent backups, one from Time Machine, the other a bootable "clone" made from Super Duper. Before erasing the internal drive, I wanted to verify that the computer could boot up from SOMETHING normal, besides Recovery. It successfully booted from the external clone. That convinced me that the internal circuitry was likely ok, but the internal drive was the reason for the no-boot. I booted into Recovery, erased the SSD, reinstalled a fresh clean OS 10.12.6, installed nothing else, and created just one user called Admin. Then I applied all available updates, including the problematic 2017-001 update, which succeeded. With the computer then booting fine, I ran Migration Assistant to migrate from the Time Machine backup all user accounts and applications, but no other settings, my theory (based on some online research) that an incompatible kext or setting in the old installation could be the cause of the earlier no-boot. Everything migrated fine, the computer appeared IDENTICAL to what it was before except for three things: drivers for our printers/scanners needed to be re-installed; I had to reconnect to the WiFi network; and I had to remind Dropbox of my name and password. Everything else worked just as before, including Mail, Microsoft Office, Adobe Lightroom, Photoshop etc.

If you have a (preferably two) reliable backup(s), this is a brute force solution but unless you have broken internal hardware or your backup is not reliable, I think it has a high probability of succeeding. The erase + clean install took about 30 minutes, and the migration from Time Machine took about two hours.

Since your OS re-install worked, you got further than I did, because in my case following that step did not result in a bootable system -- while it finished the re-install from Recovery, unlike your situation, in my case it still could not boot up. Since you have a working system, you can simply continue using it. I would certainly keep your backups current, always a good practice.

If Software Update still shows that 2017-001 update waiting to be installed, I am guessing it will install fine if you decide to try again, but you have the backups just in case. Or you can elect not to install it, but then your Mac has compromised security and future updates will most likely not be possible (including upgrades like High Sierra or whatever comes after that). For me, the failed update was not itself the problem (millions have successfully applied that update), it was simply a symptom of a different problem unique to my system that I felt should be fixed, and the clean erase/install fixed it for me.

But I wouldn't argue with your approach if you feel comfortable with it.