You will need an external backup drive large enough to accommodate the full partition of your largest operating system.
Make certain that your computer is set to 1st Boot Device=Optical drive in the BIOS. See your computer documentation if you don't know where this is.
*** I assume that you are experienced with Macrium Reflect, and have made a Reflect Emergency Restore disk that you have tested and works.
Have both drives installed in your computer, the SSD and the HDD/2TB. Power down.
> Unplug the power cable to the HDD/2 TB drive. Leave the SSD plugged in. Power up.
> Make a full backup image of your SSD/Win7 drive to your external backup drive. Test the image to make sure it is good before proceeding.
> Delete the SSD partition (IOW, wipe the drive) and create a new, full partition, then format in NTFS.
> Install Windows 10 to the SSD drive. Test it to make sure it will boot and run properly before proceeding.
> Power down; unplug your SSD/Windows 10 power cable, and plug in the HDD power cable that you previously had unplugged. Power up.
> Re-image the Windows 7 backup onto the HDD/2TB drive. Boot it and make sure it runs properly.
> Move the data that you had previously saved to your external drive back on to the HDD/Win7 drive.
Windows 10 should be your C:\\ drive and boot first. If not, you can reset the boot order easily in the BIOS. Check your manual that came with your computer for help if you don't know where to find it.
Note: you would be wise to check that you have all the chipset and other motherboard drivers for Windows 10. Be sure to install them after installing Windows 10 in case you have problems with some components.
[EDIT]: Afterthought: After imaging, you may find that the partition on the HDD/2TB drive is much smaller than the total capacity of the drive; if so, you can expand the partition size to encompass all the space using Windows 7 Disk Management or any of several other tools available.
Edited by ranchhand_, 01 October 2017 - 06:01 PM.