Don't be confused. This is not rocket science (as the saying goes)...
After you install the Win 10 OS on the SSD (following the instructions I provided you above) and you confirm that the system boots and functions problem-free, you'll then install the HDD that you disconnected. At that point the HDD will be your secondary drive.
You can then delete the Win 7 OS from the secondary drive. It's not essential but it will gain you more disk-space on that drive. NOW YOU WILL BE DELETING THAT OS FROM THE SECONDARY DRIVE, NOT YOUR C: DRIVE. YOU ACCESS THE FILES ON THE SECONDARY DRIVE THAT DRIVE MIGHT BEAR THE DRIVE LETTER D: or E: or F:, etc., etc.. ALL OTHER FILES, PROGRAMS, AND OTHER DATA CURRENTLY ON THAT NOW-SECONDARY DRIVE WILL STILL BE...