Something really strange happened. To begin I have a Crucial SSD (SATA) and an Intel SSD (NVME). Both drives have Windows 10 installed. Windows was originally cloned from Crucial to Intel. Intel drive has the Windows installation in use, Crucial was only kept as backup / failsafe. So, I was away for two days so left PC unplugged and swapped the CMOS battery. When I started the PC I got an error about missing boot device. Checking BIOS I saw that Windows Boot Manager (Intel ...) had disappeared, only Windows Boot Manager (Crucial ...) was available. Both options were available before, naturally, and I used Intel option to boot. Being unable to boot I used USB install media to get into Windows setup. I saw that indeed there wasn't an EFI partition on Intel drive, only main data partition and a 16 MB MSR partition. Not being sure what had happened I started to clean install Windows on Intel drive, erasing both existing partitions. Install started and progressed up to the point of first reboot. Again got an error "no boot device" and still no boot manager option for Intel drive. Retried installing, same story. I spent the next minutes resting my head on my hands trying to grasp the frack had just happened and happening. As a last resort decided to boot to Crucial drive and try diagnosing the Intel drive via that. So selected Windows Boot Manager (Crucial) to boot from and... Windows install I started earlier resumed from Intel drive! I now have Windows reinstalled and running on the Intel drive but somehow the boot manager of Intel drive disappeared while PC was unplugged and now the boot manager on Crucial drive actually boots the Intel drive! How the hell can something like this happen while PC is unplugged? How can a boot manager from disk A disappear and boot manager from disk B "replaces" it? Why didn't Windows setup recreate the boot manager / EFI partition on Intel drive? Smart thing to do next would be to unplug Crucial drive and re-reinstall on Intel to "force" boot manager on correct drive but tbh I'm afraid to touch the disks at this point. In short what the hell just happened?