Dell Dimension C521 - Manual (Page 15)

General Installation Guidelines
Connect the SATA hard drive to the connector labeled "SATA0." Connect the SATA CD or DVD drives to the connector labeled "SATA1" on the system board.
Connecting Drive Cables
When you install a drive, you connect two cables—a DC power cable and a data cable—to the back of the drive and to the system board.
Drive Interface Connectors
Power Cable Connector
Connecting and Disconnecting Drive Cables
When removing a SATA data cable, grasp the colored pull tab and pull until the connector detaches. When connecting a SATA data cable, hold the cable by the
black connector at each end.
1 DVD drive 2 optional floppy drive or media card reader 3 hard drive
1 interface cable 2 interface connector
1 power cable 2 power input connector