Spin-up

Spin-up refers to the process of a hard disk drive or optical disc drive accelerating its platters or inserted optical disc from a stopped state to an operational speed. The period of time taken by the drive to perform this process is referred to as its spin-up time, the average of which is reported by hard disks as a S.M.A.R.T. attribute. The required operational speed depends on the design of the disk drive. Typical speeds of hard disks have been 2400, 3600, 4200, 5400, 7200, 10000 and 15000 revolutions per minute (RPM). Achieving such speeds can require a significant portion of the available power budget of a computer system, and so application of power to the disks must be carefully controlled. Operational speed of optical disc drives may vary depending on type of disc and mode of operation (see Constant linear velocity).
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. If a machine is running and requires access to a stopped drive, then a delay is incurred while the drive is spun up. It also depends on the type of mechanism used within.
A drive in the process of being spun up needs more energy input than a drive that is already spinning at operation speeds, since more effort is required for the electric motor to accelerate the platters, as opposed to maintaining their speed.
Staggered spin-up.
In computers with multiple hard drives, a method called staggered spin-up can be employed to prevent the excessive power-consumption of spin-up, which may result in a power shortage. Power consumption during spin-up is often the highest power draw of all of the different operating states of a hard disk drive. Staggered spin-up typically starts one drive at a time, either waiting for the drive to signal it is ready or allowing a predefined period of time to pass before starting the next drive. If the power supply is able to supply sufficient current to start multiple drives at a time, that, too, is common.
Staggered Spin-up (SSU) and Power-Up In Standby (PUIS) are different features that can help control spin-up of multiple drives within a computer system or a disk subsystem. Both are defined in the ATA Specifications Standards. See Serial ATA for more information.
One feature, called Power-up in standby (PUIS) (also called PM2) is used on some Serial ATA (SATA) and Parallel ATA (sometimes called PATA or IDE) hard disk drives. PUIS requires BIOS and/or driver support to use. When power is applied to the hard disk drive, the drive will not spin-up until a PUIS Spin-Up command is issued. The computer system BIOS or RAID controller must issue the command to tell the drive(s) to spin-up before they can be accessed. PUIS can be enabled by tools such as hdparm for drives which support this feature.
Another feature, called Staggered Spin-up (SSU) is used on most Serial ATA (SATA) hard disk drives. This is more common than Power-Up In Standby (PUIS) because it does not require any special commands to get the drive to spin-up. The drive electronics waits for the SATA Data Phy (Physical I/F) to activate to spin-up the drive. The computer system BIOS and/or RAID controller or RAID driver set can delay and control when the different drives will spin-up. 
With Western Digital hard disk drives, Pin 11 of the SATA Power Interface controls whether Staggered Spin-Up (SSU) is enabled or not. Pin 11 is also used as an activity LED connection. When the drive is initially powered on, the drive senses whether Pin 11 is left floating (high or '1' logic state) or grounded (low or '0' logic state). SSU is disabled when Pin 11 is grounded. When disabled, the drive will spin-up as soon as power is applied to it. SSU is enabled when Pin 11 is left floating or driven high (high or '1' logic state). The drive will not spin-up until the SATA Phy Interface becomes active with a connection to a SATA controller or SATA RAID controller. The SATA or SATA RAID controller can control when and how many drives can be spun-up. SSU and PUIS are features that are configured in software or firmware by the manufacturer.
Information from the Fujitsu Serial ATA Interface for Mobile Hard Disk Drives whitepaper:
Note that staggered spin-up of disks is a feature of many multi-drive systems using SATA and RAID. It is not typically used on mobile platforms.