Power-Off Considerations; Load/Unload; Emergency Unload; Required Power-Off Sequence - Hitachi HTS421212H9AT00 - Travelstar 120 GB Hard Drive Specifications

11.4 Power-off considerations

11.4.1 Load/Unload

Load/Unload is a functional mechanism of the hard disk drive and is controlled by the drive microcode. Specifically,
unloading of the heads is invoked by the following commands.

Table 50: Device behavior by ATA command

Command
Standby
Standby Immediate
Sleep
Reset
Soft Reset
Hard Reset
UL = unload
Comp = complete
Rdy = interface ready
(*1) Load/Unload condition is not changed by Soft Reset
Load is also invoked as the idle command.
The specified start/stop life of the hard disk drive assumes that load/unload is operated normally, NOT in emergency
mode.

11.4.2 Emergency unload

When the drive power is interrupted with the heads still loaded, the microcode cannot operate and the normal
5V power is unavailable to unload the heads. In this case, normal unload is not possible, so the heads are unloaded
by routing the back EMF of the spinning motor to the voice coil. The actuator velocity is greater than the normal
case and the unload process is inherently less controllable without a normal seek current profile.
Emergency unload is intended to be invoked in rare situations. Because this operation is inherently uncontrolled, it
is more mechanically stressful than a normal unload.
A single emergency unload operation is more stressful than 100 normal unloads. Use of emergency unload reduces
the start/stop life of the drive at a rate at least 100 times faster than that of normal unload and may damage the drive.

11.4.3 Required power-off sequence

The following are examples of problems which can occur when power is removed on most drives at an arbitrary
time:
• Data is lost from the write buffer.
• If the drive is writing a sector, a partially-written sector with an incorrect ECC block results,
the sector contents are destroyed, and reading that sector results in a hard error.
• Heads may land in the data zone instead of the landing zone depending on the design of the
drive.
Response
UL -> Comp.
UL -> Comp.
UL -> Comp.
Response
Rdy (*1)
UL -> Rdy
Travelstar 4K120 Hard Disk Drive Specification
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