BIOS change puzzle

[pgoelz]

Its kind quiet here these days so here's a puzzler.

I now have two X61S. I'll call them #1 and #2. #1 is my old machine with Middleton BIOS, Windows 7 and Windows 10 on an SSD, #2 is a recent Ebay purchase (no HD) and is the 1.8GHz variant.

For initial testing on #2, I used the original hard drive from #1 before I upgraded it to an SSD. That HD has Windows 8.1 and Windows 10 on it. Worked great, although I did have to re-activate Windows 8.1.

In preparation for swapping the SSD from #1 to #2 I updated the BIOS in #2 to Middleton. After loading all BIOS defaults, I got an "inaccessible boot device" error on the HD when trying to boot to Windows 8.1 (the default OS). Huh? It worked before the BIOS update. Since I've "been there and done that" before, on a hunch I switched to "compatibility mode" and it booted OK. Not sure what happened... it was booting just fine with the old BIOS and the HD worked fine in my old X61S with Middleton BIOS and that machine is definitely in AHCI mode. Decided to leave it for the time being since the goal was to get the SSD into #2.

So..... I set #2 back to AHCI mode, pulled the SSD out of #1 and put it in #2. It booted fine although I did have to re-activate Windows 7.

I then put the HD into #1 (Middleton and AHCI mode, and where the HD originally came from) and the boot stalled at the Windows logo. In start up repair mode, it offered to boot into Windows 10 so I let it. That actually booted OK. On the next boot, the usual OS selection menu came up and now it boots Windows 8.1 AND Windows 10. Huh?

What happened? I'm assuming Windows lost track of the boot loader and maybe booting to Windows 10 made it find it again.... it definitely COULD access the HD because the Windows logo came up and the HD started accessing before it died.

So.... everything is fine and everyone is booting OK in AHCI mode. I'm just wondering what happened

Paul