Need help with my hard drive

So back when I got this pre-built pc, I was told it had 120GB SSD and a 500GB HDD. When I took my pc to tech guy to get the power supply changed, he told me one of my hard drives wasn’t plugged in and that he could do that for me. Well I just now realized that I don’t think the 500GB HDD hard drive was ever plugged in/properly worked. My storage has only ever showed 120GB of storage so the SSD is the only one working. I just checked the hard drives and they both seem to be plugged in. I was debating on if I should buy another SSD with more storage and replace the HDD drive that isn’t working or if there could be something else I could do to try and make the HDD drive work. I haven’t really messed with the wires because I don’t know what I’m doing, but if you all recommend me to do something I can try. If I was to replace it, I don’t know which one would be the HDD drive. They both look similar, however one has another small port beside the DATA port, while the other doesn’t. Would that one be the HDD drive? Sorry if all of this seems dumb, I really just know nothing about computers haha.