Final answer:
Pie-shaped wedges on the surface of a hard drive are known as sectors, which are the smallest storage units on a disk where data is recorded.
Step-by-step explanation:
The pie-shaped wedges on the surface of a hard drive represent sectors. A hard drive's surface is divided into circular tracks, and these tracks are further split into sectors. The sector is the smallest storage unit on a disk and is commonly a few kilobytes in size. This is where data is read from and written to by the hard drive's read-write head, akin to the thin stylus mentioned in the image description from Figure 12.28. A platter is the disc itself, a cylinder is a stack of tracks lined up across multiple platters, and a track is one complete circle of data.