win 10 will replace your win 7 install, you cannot keep using it. Registering win 10 with the key means you are sacrificing it for a win 10 key, unless you decided to swap back to using the win 7 install only before the end of the next 4 weeks is up.
So once ssd has win 10 on it, reconnect hdd and then copy everything across you want to keep. You could keep win 7 there as backup for a few weeks, in case you decide to go back to it, but after the month you best format the partition win 7 was on and use it as storage.
it won't touch the hdd if its dc during the install.