If you reinstall which is the best choice then once you activate on the SSD, if you start up the HDD it will soon ask you to reactivate it which you should not do to use the activation on the SSD. If you remove the key from HDD it will go non-geniune. It's best to only keep it slaved to the SSD to pull data off of it then wipe it, or keep it as a backup which you could later reactivate if you need to go back to it.
If you decide you're ready to wipe the HDD, open an Elevated Command Prompt
from Win7 on SSD and run Clean command: Clean and Clean All with Diskpart Command
then you can create and format new partitions in Disk Management: Partition or Volume - Create New