When a disk drive is operating, the disk is rotating at constant speed.
To read or write, the head must be positioned at the desired tack and at the beginning of the desired sector on the track.
Track selection involves moving the head in a movable-head system or electronically selecting one head on a fixed head system.
On a movable-head system, the time taken to position the head at the track is known a seek time.
Once the track is selected, the disk controller waits until the appropriate sector rotates to line up with the head. The time it takes to reach the beginning of the desired sector is known as rotational delay or rotational latency.
The sum of the seek time, (for movable head system) and the rotational delay is termed as access time of the disk, the time it takes to get into appropriate position (track & sector) to read or write.
Once the head is in position, the read or write operation is then performed as the sector moves under the head, and the data transfer takes place.