Yeah, SATA 3Gb/s is SATA II. The old HDD, you can keep plugged in, but it may slow down the system nevertheless and I would not trust it with anything important. If you don't mind losing whatever on it (may happen tomorrow, or not in a year, one can never know for certain) then you can keep it. So if you see any issues, remember to remove it first thing you do, and see if it solves the issue.