You could choose to do a clean install, and I am assuming windows 10 here only, on the SSD. Also assuming that you windows 10 us legal and valid, then that key is actually kept on Microsoft servers, so they will activate that without issue. Then you can plug the hdd back in afterwards.
The problem here is that you will have lost all your settings, configuratons, user account setup, etc. etc. that you were used to having, and will have to manually re-establish all that, re-install all the applications, including such basics like MS Office, Chrome, anti-virus etc. Additionally on your HDD (with the 1TB of data) will have wasted partitions which were created for windows recovery etc. on the HDD. It may not be a big deal, since you might...