Spin-up of hard disks generally occurs at the very beginning of the computer boot process.
However, most modern computers have the ability to stop a drive while the machine is already running as a means of energy conservation or noise reduction.
Avoiding simultaneous spin-up helps keel the inrush current under power supply limits.
[2] SSU allows a SATA host bus adapter to spin up hard drives in a sequential manner.
[4] On connectors designed around older SATA revisions, the pin is connected to ground, disabling the feature.
[8] The computer system BIOS or RAID controller must issue the command to tell the drive(s) to spin-up before they can be accessed.