Mac won't boot after Sierra update

Hi Paco. Did you ever resolve your problems in the end ? I read your OP and it echoes the recent experiences on my Mac Mini 2011. I have upgraded RAM 4GB->8GB, switched the internal failing (boot) hard drive for a Samsung 500GB SSD and tried clean Internet Recovery reloads (High Sierra 10.13), Time Machine (revert to El Capitan 10.11.x) and after another Internet Recovery have somehow (?) got Sierra 10.12.6 loaded. All my efforts still result in instability and not able to boot normally without Restart looping, or if it has managed to get to the User Login it doesn't stay logged in for long. I did see @1hour between restarts and the system (running High Sierra at that time) was flying performance-wise ! I've tried all the advice seen on the web that you listed (and more) and now appear to have a very stable system running in Safe mode but am wondering how to diagnose what the differences are from Safe to Normal booting. Starting up with Verbose on the text flashes up and off the screen so fast I can't read it and even trying to film on my iPhone it goes blurry/smudgy so isn't clear to read. Pretty sure it must be a driver, .kext or similar that the system tries to load for normal boot that causes the problem but don't know how to identify. Currently have a pretty "vanilla" Apple only system in case I have to stash it all yet again - don't want to waste time and effort recovering from TM or possibly introduce more issues ! Thanks in advance.

Thanks. Might be about the only method I have yet to try - assuming it's the local disk Recovery as against Internet Recovery.... I tried that as my boot drive was failing and couldn't read anything and hence ended up at High Sierra 10.13 !

Oh well, in for a penny..... Here goes ! See you on the other side.....

The FIPS power on self test failed, possibly due to an incompatible aftermarket SSD or its firmware. The latest Apple documentation regarding FIPS is available here: Apple FIPS Cryptographic Modules v7.0 for macOS Sierra 10.12 - Apple Support.

If it continues to occur with a compatible SSD or conventional hard disk drive then it is a hardware fault that only Apple will be able to fix: Contact Support

If you use Time Machine to back up your Mac, you can easily revert to a previous version of macOS if you experience trouble after installing an update: macOS Sierra: Revert to a previous macOS version.

If so, is external usb install my best shot?

I'm not quite sure what you mean by that, but read How to set up and use an external Mac startup disk - Apple Support.

I had the same issue with my Mac...Wont boot after Sierra Update! I did what Bobharris said below and it worked perfect.

Brandon

There is no way to bypass macOS's self-tests. macOS upgrades often include corresponding firmware upgrades for various hardware components. Did you expect the aftermarket SSD and its resident firmware to be compatible with all future macOS upgrades?

I would try re-installing macOS over top of your current installation via the Recovery Partition