Epson 386SX - Manual (Page 81)

Because a hard disk consists of two or more platters stacked on
top of one another, it has four or more sides with many more
tracks per side than a diskette. (The number of tracks depends
on the capacity of the hard disk. You do not need to know how
many sides and tracks your hard disk has.)
A disk is further divided by sectors. To understand what a sector
is, picture the spokes on a bicycle wheel radiating from the
center of the wheel to the tire. The space between one spoke
and the next is like a sector on a diskette; the lines dividing the
sectors cut across the tracks. (See the figure below.) A diskette
can have 8, 9, 15, or 18 sectors per track. The number of sectors
on a hard disk depends on the type of hard disk.
sector
Using the Equity 386SX 3-11