Spin-up is when a disk in a disk drive speeds up to the required revolutions per minute for effective writing to or reading from the disk. Conventional hard disk drives have one or more platters that revolve mechanically on a spindle while reading and writing elements alter the magnetic surface of the disk.
Techopedia explains Spin-Up
One common reference to spin-up is the use of the term spin-up time to analyze the time that it takes for the disk to go from a dormant state to a readable or writable state. Spin-up time is influenced by many factors, including the required revolutions per minute, which range from around 3,000 to 4,000 for a personal device to up to 15,000 revolutions per minute or more for sophisticated servers.
Because spin-up requires a major portion of total power output for a computer or device, engineers have tried to limit the required power and the burden of spin-up on a specific device model. This includes efforts to stagger spin-up, or another process called power up in standby, where a RAID controller might control this process for a given drive or set of drives. Specific manufacturers have also developed proprietary technologies for optimizing spin-up and delivering low-energy solutions for the tech market.