Power-up in standby (PUIS) or power management 2 mode (PM2; Western Digital specific) is a SATA or Parallel ATA (aka PATA) hard disk configuration which prevents the drive from automatic spinup when power is applied. The spinup occurs later by an ATA command, only when the disk is needed, to conserve electric power and to avoid a power consumption peak caused by a simultaneous spin-up of multiple disks. Typical power consumption of desktop drives is several watts when active/idle and several tenths of a watt when in standby.
PUIS requires corresponding BIOS support. If PM2 is enabled on the drive but not supported by the BIOS, the drive will not be detected by the system or detected as zero in size. PUIS is typically only supported on RAID controllers.
PM2 can usually be enabled by jumpering on the drive but can also be configured by other means (configuration sector) using manufacturer specific tools.
ATA Commands for PUIS Usage
ATA Task File Settings
Feature = 0x06 - Enable PUIS
Feature = 0x86 - Disable PUIS
Feature = 0x07 - Spin up drive after power on
Command = 0xEF - Set feature