How does a second hard drive in a computer work?
Overall, a secondary hard drive works the same as a primary hard
drive. In the case of PATA drives, the secondary hard drive
operates as a "slave" drive. It disables its controller and lets
the primary hard drive control it. This is determined by jumper
settings on both drives and/or the cable position.
While SATA drives are modeled after PATA drives, SATA drives
operate on independent channels, and thus every SATA drive is a
"master" drive. Their order or addressing are all dependent upon
which socket they are plugged into.