Want to add Windows 7 to my Windows 10 computer so I can run some of my old games that are not compatible with 10. So far I've had bad luck. Install starts but then fails and I lose the bootloader for 10. Had to do recovery install of 10.
firstly, did you partition your C drive to make room for the 2nd OS? I don't not recommend dual booting.....that's is 2 OS's on one HDD. I recommend a mutlboot setup.....an OS on separate drives.
Install the 2nd HDD and remove cables from 1st drive. This prevents the MBR's from being installed on the C drive. So your MBR will be on both drives. Then install 2nd OS on 2nd drive. Once up and running and fully updated......power off. Then reattach cables to 1st drive and boot up. By default Windows 10 should boot from the C drive....which is normal. To boot from 2nd drive.....on a reboot, start tapping the F12 key. This will bypass the BIOS and take you straight into the boot menu and from there just select your OS you want to boot from.