IBM's 'Active Protection System' is a combination of software and hardware, the hardware itself residing on the laptop's motherboard.
However, this doesn't mean that APS will be compatible with any hard drive you use. Hard Drives (which don't have 'free-fall' technology onboard) will only work with IBM's APS if they meet certain requirements - those being Head Unload Ramp Technology and customised firmware to take advantage of APS 
I'm unsure of what drives work with IBM's APS technology. What hard drive do you currently have? Purchasing a hard drive which is made by the same manufacturer (and perhaps is the same model) as your current may be your safest bet. Alternatively, purchasing a hard drive with 'free-fall' technology built in is another solution. Seagate Momentus 7200.2 hard drives, for example, has onboard 'free-fall' technology.
To be certain, I'd get in contact with IBM technical support.