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.
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
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.