[ltp] Does hdaps [APS] work on non-Thinkpad hard drives?

[linux-thinkpad@linux-thinkpad.org]

Hello,

My understanding of APS [Active Protection System] on supprting Thinkpad laptops is that the mechanism detects movement and when it reaches a critical level, the APS, via software, parks the drive heads.

However, according to the ThinkWiki problem page [1], there are problems with non-Thinkpad branded hard drives. To quote:
Additionally, the following ATA commands are implemented by ThinkPad-branded drives but are not supported by most other laptop drives: 
UNLOAD IMMEDIATE (needed for the Active Protection System) 
SECURITY FEATURE SET (needed for disk password protection)

Is this correct? How can other hard drives not be supported by APS if all it does is park the heads, which every hard drive can do?

Is UNLOAD IMMEDIATE something that exists in only a few hard drives? And is it even required for APS to function correctly as the ThinkWiki suggests?

[1] http://www.thinkwiki.org/wiki/Problem_with_non-ThinkPad_hard_disks#Problem_Description

Thank you.