Looks like many people had this issue in the past with Win7

Boot Windows 7/8 from GPT on BIOS system : No hybrid MBRs or DUET! - Windows 7 Help Forums
...

Hi everyone!

I was wondering if there is an easy way of installing W10 in larger disks (+2TB) on systems with no UEFI Support

Right now my HDD is formatted with MBR, windows 10 is installed in...