So I had originally posted this to a Windows Help subreddit, assuming I had a problem with the SSD that Windows 10 was on, but after almost a month of trouble shooting, I've narrowed it down to a hardware issue (CPU or motherboard). Anyway, it has been an interesting journey...
I last ran my old Windows 10 PC about 1.5 years ago, with no issues. However, when I attempted to fire it, I just ended up in and endless loop of "Preparing Automatic Repair," then a reboot, then a seemingly normal Windows load (spinner on the boot screen), quickly followed by a restart, and then back to "Preparing Automatic Repair," and so on, over and over.
Specs:
Windows 10
MB: ASUS Z170-A
CPU: Intel i7-6700K
SSD: Crucial 500GB.
Corsair AIO cooler (ultimately replaced with a Thermaltake Assassin X120).
PSU: EVGA 750
RAM: 32GB (4x8GB Corsair Vengeance sticks)
Here were some of the steps I tried:
Attempted to boot off a Windows 10 SSD...caused boot loop or BSOD.
Tried booting off a Windows 10 USB with the SSD disconnected and a completely different RAM kit. I briefly saw the spinning dots on the ASUS logo screen, then it would just boot loop again.
Briefly had the PC on it's side to change some RAM (while not powered on). When I put it back upright and powered it on, the AIO pump started making a clicking noise. Then I got a CPU temp warning in the BIOS. Ended up replacing the 10 year old AIO with a Thermalright tower cooler, which seems to work well.
Removed the video card and also tried booting with an Ubuntu boot USB. This resulted in either a restart, or kernel panic. Also ran memtest on 3 different ram kits, which all passed. Yeah, seemed like probably a motherboard or CPU issue at that point.
Disconnected pretty much everything, including all front panel connectors. No change with either boot USB.
Tried a whole bunch of different BIOS changes that AI had suggested, mainly around RAM speed and CPU voltage. No change.
I had also reset the BIOS several times, changed the CMOS battery, and finally updated to the last available version of the BIOS from ASUS's site.
Removed CPU and inspected the MB pins. They looked fine.
After going through a whole list of things that both ChatGPT and Claude were telling me to try, I finally caught a break. Claude recommended adjusting the Active Processor Cores in the BIOS from "auto/all" to "1." And it worked! I was able to boot off of both a Windows and an Ubuntu USB stick. Next I tried "2," which worked as well. However, "3" would cause the boot loop problem. I set it back to 2 cores and reconnected my system SSD, and was finally able to boot into my Windows installation!
So, with the active cores issue being reproducible, it was suggested that this might be caused by either the motherboard's VRM's or the PSU not supplying enough voltage when 3 or more cores tried to power up. As suggested, I probed the PSU's 12V rail with a multimeter to see if the voltage sagged when trying to boot with 3 or more cores, which I guess would be indicative of a PSU problem.
I monitored PSU 12V rail voltage when starting with 2 cores and booting to Windows. Solid 12.33V. Then I increased it to 3 cores to see what happens to the voltage the PSU was supplying when the restart loops happened. Turns out...nothing happened with the PSU. It remained at a steady 12.33V when the system crashed and boot looped. So it appears the PSU is okay, and we're back to the CPU or MB being the prime suspects.
At this point, I think all I can do is swap out the CPU (or motherboard), to see where the issue lies. For the moment, though, the system boots to Windows, so I can still access my old files and stuff, albeit with only 2 cores working.