Switching Hard Disk Causes Boot Error

I've upgraded one of my machines to Windows 10. I put in a new SSD and left the old Windows 7 drive alone, just unplugged. I wanted to leave it intact in case I decided to downgrade. I should mention that I did an upgrade install of Windows 10 by installing Win 7 to the SSD first.

The problem is this: Today I realized that I needed to use something briefly from the Windows 7 drive which did not work on Windows 10 yet (replacement program arrives on Monday). I unplugged the Win 10 drive and plugged in the Win 7 drive and rebooted.

It would not boot up. It got halfway into the colored fly-in, gave a brief BSOD which I couldn't read, and then rebooted. It did this multiple times before I ran Startup Repair. Startup Repair didn't help at all, but it did tell me that there was a driver error. I tried System Restore and safe mode and neither worked. Safe mode just stalls at "ClassPNP.sys" and then BSODs and reboots.

Nothing has changed in this machine hardware wise, so I don't get why there is now a driver error. It's not like the motherboard would remember the Windows 10 configuration. Going back to Windows 10 works fine.

Any ideas, anyone? I'm at a loss.

With both drives installed and booting into what ever operating system works take a look in Disk Management and see if things look as they should.

Good idea, Jack. I'll have to image the Windows 10 drive first. The last time I hooked both drives up at the same time it fragged the Win 10 bootloader and I had to reinstall.

I remembered after my original post that I had a clone of the Win 7 drive from a couple of weeks ago. Plugging it in gave me the same error and BSOD. System repair says it's also a driver or update error like the first drive, but how can that be? There have been no changes to the hardware since I took those drives offline. I'm completely baffled.

Well, I fixed it, but the solution is so stupid (or rather I am), that I hate to admit it.

Turns out that the original drives were set for IDE in BIOS, not AHCI. I set the new SSD to AHCI and forgot to change the BIOS back when I was trying to boot into the old drives.

Ok, hiding my face now...