Firstly, are you adding a second drive? If so, have you done the usual - checked that both drives have the header cable correctly aligned on the pins so that it is not "off by one pin" or "off by one row"? Is the second drive correctly jumpered so that it knows it's a slave?

Is the other end of the cable correctly onto the header on the main board, again, not OBOP or OBOR? Is the cable correctly orientated on both drives and the main board connectors? Is the cable in good condition? Do you have a pale grey with red stripe cable, or dark grey with black? Can you hear them spin up?

What happens if you remove the cable from this new drive, does the system function correctly again?

What are the types of your two drives - are they the same manufacturer?