Thinkpads Forum

HD-APS requires a hard-disk with head unload ramp technology and also support on the hard-disk firmware to unload heads without flushing the disk cache. This is required, because as soon as the APS system detects a shock is imminent, the system has less than 500ms to prepare for the shock.

Unloading heads without flushing the cache is done using the optional Unload Immediate feature of the IDLE IMMEDIATE ATA command. It finishes whatever sector write is in-flight, and immediately moves the heads to the unload ramp. Without this command, hard-disk APS cannot be trusted, as disks with big caches can take a lot of time to write it all to disk.

So far, only hard-disks with IBM APS firmware, as well as the consumer Fujitsu HV2060AH/MHV2100AH/MHV2120AH HDs have been found to implement all the necessary functions. Head unload technology is reasonably common in modern laptop disks, but the APS firmware is very rare in regular consumer products. Please note that newer Apple notebooks also support APS, so it is somewhat likely that their disks also support unload immediate or a similar feature.