Spin-up refers to the period of time taken by a traditional (non-solid state) hard disk drive to accelerate its platters from a stopped state to an operational speed. The required operational speed depends on the design of the disk drive. Typical speeds of 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.

Spin-up 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.

A drive in the process of being spun-up consumes more electricity than a drive that is already spinning at operation speeds, since the electric motor must work harder to accelerate the platters as opposed to maintaining the speed of the platters.

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 from resulting in a power shortage. Staggered spin-up typically starts one drive at a time, waiting either 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.

It requires a feature called Power Up in Standby (also called PM2) in both the hard drives and the computer's BIOS, as this option keeps the hard drives from spinning up when receiving current, waiting for the staggered spin-up

Information from the Fujitsu Serial ATA Interface for Mobile Hard Disk Drives whitepaper :

Staggered spin-up is a simple mechanism by which the storage subsystem controller can sequence hard disk drive initialization and spin-up. Having this feature not only provides greater reliability, but it allows the system to avoid power surges if all of the HDDs spin up simultaneously during system power up (in a multi-drive environment). Another benefit to having staggered spin-up is the use of more cost-effective power supplies, which prevents power supply damage and system brownouts.

Note that staggered spinup of disks is not confined solely to mobile drives, and is a feature of many multi-drive systems using SATA and RAID.

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