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#11
Your original problem is that you have Windows 10 installed in legacy BIOS mode, which means it is installed on an MBR disk with an active NTFS partition as the system partition the computer boots from. When you disable CSM mode (which is legacy BIOS emulation), your computer shifts to booting in UEFI mode - which is a requirement for secure boot. The majority of computers cannot boot from an NTFS partition in UEFI mode, most computers need a FAT32 partition to boot from in UEFI mode - and, in addition, the system partition must also contain the proper files to boot in UEFI mode, which your original legacy BIOS install of Windows 10 likely does not have.
I don't see a problem with the motherboard. The problem is that your current install of Windows is not compatible with UEFI booting which you are trying to switch to.