If the OS was originally installed on a system with CSM ON (Legacy on), then it has to stay on unless you do MBR2GPT and switch to UEFI boot. Once it's in UEFI, mode, you're not out of the woods.
If the OS was installed in RAID (Intel RST) mode, then Windows needs the RST driver installed. You'd need to install it from safeboot. However it does not appear to be getting that far, and it's more likely the UEFI/Legacy switch that you need to toggle.
The more or less correct way to solve this is to unplug all drives but the drive you want to boot from, go back into the BIOS and select the only drive, ensure Windows boots, and then plug everything back in. Then if you want to switch to UEFI mode proper, follow the correct steps to do so by making sure the RAID or AHCI mode matches the driver in the OS, and then do (MBR2GPT, Switch BIOS to UEFI mode, turn legacy CSM off.) Under UEFI, if there's a UEFI boot partition on the drive, it will be updated in the BIOS the first time it boots in UEFI mode and you don't have to fiddle with it further. If you continue to boot it in legacy mode, then other drives plugged in can "pre-empt" the boot drive depending on the boot order. So leaving a disc or usb drive plugged in that is marked "bootable" but lacks a working bootloader will do what you've seen if there are no bootable drives.
MatchewR reacted to Kisai in SSD is being picked up by BIOS but it won't boot
