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Setup is configured in the BIOS rather than by rearranging drive positions inside your PC case. Find the specific key for accessing BIOS on your motherboard—often the Delete key works. Alternatively, press F8 repeatedly during startup to see a menu of drives for selection. To alter the default boot operating system in Windows 7, refer to the steps in this resource: https://www.sevenforums.com/tutorials/22...ot-os.html
You attempted to start the HDD before the SSD, adjusted the boot sequence in the BIOS, but Windows 7 still launched.
Some MBs have a hot-pluggable option you can set for individual SATA ports. If you don't have an external option to plug in the Win7 drive then you could set a SATA port to hot-pluggable and plug the Win7 drive there. Remove only the SATA data cable for the Win7 drive. Boot into your Win10 drive then plug in the SATA data cable to the Win7 drive. Wait, I just re-read your post, why don't you just do a fresh Win10 install on the SSD that currently has Win7?