The Memorex disk was " hard-sectored ", that is, it contained 8 sector holes ( plus one index hole ) at the outer diameter ( outside data track 00 ) to synchronize the beginning of each data sector and the beginning of a track.
12.
Early 8 inch and 5? inch disks had physical holes for each sector and were termed " sectored " disks had only one index hole, and sector position was determined by the disk controller or low level software from patterns marking the start of a sector.
13.
These early drives read only one side of the disk, leading to the popular budget approach of cutting a second write-enable slot and index hole into the carrier envelope and flipping it over ( thus, the flippy disk ) to use the other side for additional storage.
14.
Another LED / photo-transistor pair located near the center of the disk detects the " index hole " once per rotation in the magnetic disk; it is used to detect the angular start of each track and whether or not the disk is rotating at the correct speed.
