I have a very weird issue.

I had succesfully installed Windows 10 from an ISO, off flash card actually, to my regret.

So it was a clean install. Boots fine, everything works except I have a slow harddrive for some reason, I get stalling harddrive. Frequently and particularly at the start of booting.

I do some stuff to attempt to test another PCI RAID card. My idea is: maybe my harddisk will behave differently if on a different controller.

But of course Windows has a problem with that. It cannot boot, because bootmgr is now corrupt. It cannot find the boot device (2nd partition on the disk).

So I intend to see if "Startup Repair" could fix that and I boot it off of the SD card. While Startup Repair is running, quite quickly the machine suddenly reboots. Never happened before.

I try to boot again but now I get errors. They are differently worded but come down to BOOTMGR trouble.

So I'm thinking, it must have corrupted my harddisk. I rage. Eventually I learn that it even gives that error when no harddisk is present!!!.

So I am getting a BOOTMGR error when I do not even have any harddisks connected. I am just trying to load the installer.

Next I burn an ISO with the same Windows 10 only this time it is just the x64 version, whereas the SD card held both x86 and x64. Luckily enough, it boots.

But I'm a bit foolish and I attempt to run Startup Repair again. This time it finishes but very quickly it tells me it cannot repair my PC. However I cannot continue to the installer, so I have to reboot. I reboot and attempt to load the DVD again.

BOOTMGR image is corrupt. The system cannot boot. Exactly this right:

So to recap now (probably) both the Flash Card and the DVD featuring basically the same version of Windows 10 refuse to boot.

The Windows 10 installation that I still have boots fine

Any Windows 8 DVD boots fine
Any Windows 7 DVD boots fine
My installed Linux system on another disk boots fine

The only things that will not boot are the two Windows 10 installer images from which I ran STARTUP REPAIR.

Almost as if they wrote something to the BIOS

This is a non-UEFI system. It is a motherboard from 2009. Resetting the CMOS doesn't help. Removing the PCI controller doesn't help. Removing a bunch of other stuff doesn't help either.

To the Windows 10 installers I have used, this motherboard is now dead. To all other OSes and DVDs, it is not.

The only thing I can really do now is to load a Windows 8.1 DVD and hope that bootrec /rebuildbcd will do anything (good).

What on earth is happening and what can I do?