You will need 2 torx screwdriver bits/screw drivers.

A Torx 10 and a Torx 20.

I believe you also need a T15 to remove the hard drive from its cradle.

i used a torx10 perfectly well for upgrading my hdd.

Yeah, I recently lost my T15 and I used the T10 in a pinch. The T15 is a better fit, but the T10 still works fine with no stripping or anything.

there are torx sets that come with all the sizes of the torx.

i suggest that.

I bought a Black & Decker multi-bit magnetic ratcheting screwdriver kit for $10 at Home Depot. Comes with Torx 10, 15, and 20. Plus lots of other handy screwdriving stuff.

In a pinch i use hex keys for the inner screws, you just have to have a good set or you will strip em.