Three Pentalobe screws secure the battery to the upper case. They can be removed with this special driver.

If you don't have a Pentalobe driver, a 1.5 mm flathead screwdriver can be used in a pinch. Be sure the head of your flathead screwdriver fits snugly across two of the five "points" of the screw head before trying to break the screw free, as a loose fit will easily strip the screw head.

If the head of your screwdriver fits too loosely, find a bigger bit and file it down until it fits snugly before proceeding.

You do not necessarily have to follow steps 3-7 to remove the battery in order to replace the hard drive. However, it is recommended to remove all power sources from electronics before working on them.