Power-up in standby or power management 2 mode (PM2)(Western Digital specific) is a SATA

Sata

Sata is a traditional dish from the Malaysian state of Terengganu, consisting of spiced fish meat wrapped in banana leaves and cooked on a grill.It is a type of Malaysian fish cake, or otak-otak...

 or Parallel ATA (aka PATA) hard disk

Hard disk

A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...

 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.

PUIS requires corresponding BIOS

BIOS

In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....

 support. If PM2 is enabled on the drive but not supported by the BIOS

BIOS

In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....

, the drive will not be detected by the system or detected as zero in size. PUIS is typically only supported on RAID

RAID

RAID is a storage technology that combines multiple disk drive components into a logical unit...

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