Power down the computer and unplug any power cords. Locate and remove any screws or clamps that hold the computer case or panels.
Remove the computer case or panels to access the interior of the computer case. Unplug the IDE ribbon cable and 4-pin power connector from the current hard drive.If the case contains a removable hard drive bracket, remove this bracket from the computer case to make for an easier installation.

Based on the documentation that came with the hard drive or any diagrams on the hard drive, set the jumpers on the drive to enable it to be a Slave drive.

Slide the hard drive into the drive bay and fasten it down. Some cases use a system of slide rails that mount onto the drive for easy insertion and removal from the drive bay.

If the case uses a removable drive bay bracket, return the drive bay bracket to the inside of the computer case.

Attach the IDE cable connectors from the ribbon cables both into the old hard drive and the secondary hard drive. The connector furthest from the motherboard (often black) should be plugged into the primary hard drive. The middle connector (often gray) will be plugged into the secondary drive. Most cables are keyed to fit only in a specific direction on the drive connector but if it is not keyed, place the red striped portion of the IDE cable towards pin 1 of the drive.

Locate the 4-pin power connectors from the power supply and attach them to the power connectors on the hard drives.