Question: Wondering because I have salvaged a hard drive from an old pc. I already have a Hard Drive and an ssd. Is there anything I need to know?

Answer:

Wondering because I have salvaged a hard drive from an old pc. I already have a Hard Drive and an ssd. Is there anything I need to know?

As long as your motherboard has sufficient ports, absolutely.
My main system has 7 drives. All SSD, but that is irrelevant. SATA SSD is no different than SATA HDD, other than speed.