Working build died Win7/Comet Lake - No Boot Device Found

Would love any help possible with this

MSI Z490-A Pro

Did a Win 10 install on an SSD partition just in case to start off
Attempted Win7 installs off USB, all were hanging.
Flashed BIOS to give me option of CSM instead of UEFI
Win7 installed correctly on separate SSD, had to use MBR not GPT
Installed drivers, programs, rebooted etc all was fine. Had no undetected hardware
Wiped first SSD (that had windows 10 install on it) and installed a few programs on it

I did also pull over basically everything from my username\roaming profiles to the new C:\ from my old Win7 install (5600k)

Left it overnight, came back this morning and now I get No Boot Device Found!

Unplugged all other SATA devices, so just the Win 7 SSD is in, same issue.

Booted off a Hirens USB, it’s detecting the SSD perfectly, can browse it etc

Tried a bcd repair, “the requested system device cannot be found” even though both disk manager/diskpart can find it.

Easy BCD can’t find a BCD.

There’s no 100mb system partition created from that Win7 install.

Currently building the Win7 bootable USB again, but I would welcome any advice possible!

@pevergreen :
Welcome to the Win-RAID Forum!
It is always a good idea to unplug all other HDDs/SSDs except the target one before you start with the OS installation. Otherwise you risk, that the OS Setup will put the boot sector onto the wrong disk drive.
Good luck!
Dieter (alias Fernando)

Good call, I can’t be sure I did that so will ensure I do if I end up not being able to repair the current installation and instead go with a fresh one on the same drive

i suspect fernando is correct, modern windows OS’s ( all since xp i think) work off a unified boot loader where if at the time of install it detects another windows install on any connected drives it will modify the boot parameters on the the existing windows boot drive instead of writing a new boot partition to the drive you are installing secondary windows install on to ( with windows 10 are your primary and windows 7 as the secondary in your case) i believe this was done to ensure uniformity of the boot menu when dual booting regardless of which OS you boot into last and to not confuse the bios/uefi as to which dive to boot from which can cause boot issues in rare cases.

This is why i suspect you nuked you boot partition when you formatted the windows 10 drive and you could not longer boot once you restarted the system. i would recommend in the future anytime you are installing a OS to disconnect all drives from your system expect for the drive you are installing to and the drive media (either usb or dvd) that your installing from in order to prevent windows from writing its boot partition anywhere other than the drive your installing windows on. sadly this has become much more of a pain in the butt with m.2 drives as it means you now have to tear your entire machine apart just to install/remove them.

hope you found this information useful.