Quote Originally Posted by Fire$torm View Post
Almost every laptop made in like the last 5~10 years or so has one. The sensor is used to restrict the laptops HDD from doing reads/writes during excessive or sharp movements. This keeps the user from killing the HDD.

The problem is that most lappy manufactures are so anally secretive about how they implement hardware that they refuse to release the data needed to access the sensors via software. Hence the need for the external sensor.
...or more to the point, some of us don't have laptops. Also, it's more accurate to have the USB sensor because you can give it the proper x,y,z orientation and place it somewhere more stable/permanent.