imac late 2012 with 2 installed drives; main ssd has died/dying, how do i/can i make 2nd 3 tb (hdd) drive main/boot drive and be recognized as such.

I bought a used imac 27" late 2012 which has a 125 gb SSD as main boot drive and a secondary 3 tb hdd that is a data/itunes drive. I have successfully made the hdd a bootable drive running High Sierra. The main boot (ssd) is failing/has failed per disk repair. I want to make the secondary 3 tb hdd the main boot drive and, for all intents and purposes, permanently unmount the sdd. Is this possible. I need to do so so I can use boot camp to make the hdd half mac and half windows (some work programs require windows to run). Is there a way to disable/permanently unmount the ssd so it is not recognized? Cannot use bootcamp as it says the ssd does not have enough room to partition, but it is the hdd i want to partition and install windows on? I've tried everything I know and cannot really find a straight answer. Thank you so much.