If your PC has two SATA drives installed and running, which one is the master?

There is no need for a master or slave distinction with SATA drives because they each connect to the motherboard with their own SATA cable. Two IDE/PATA drives, on the other hand, could share the same data cable, but only one could use it at a time and thus each required a distinct name. In fact, by the time ATA-2 was approved in 1996, the spec referred to the drives as device 0 and device 1, despite master/slave being more commonly recognized.

When installing two PATA drives, jumpers on the back of the drives let you designate them a master or slave, the former of which had no priority over the latter and motherboards could also typically boot off either drive based on user preference. "Cable Select" was also a jumper option that could be chosen on the back of both drives to let them automatically set themselves ...