Windows boot error - A data disk is currently set as active in BIOS

An error encountered when trying A's Windows install -  "A data disk is currently set as active in BIOS. Set some other disk as active or use the DiskPart utility to clean the data disk, and then retry the restore operation. (0x80042406)"

https://answers.microsoft.com/en-us/windows/forum/all/a-data-disk-is-currently-set-as-active-in-bios-set/eb931d76-9d23-424c-ba40-8fcd20633125

Apparently this means that the boot partition is borked and we have to boot with a System Restore disk and then restore the image.