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The head actuator is the device that physically positions the read/write heads over the correct track on the surface of the disk. Floppy disks generally contain 80 tracks per side. The actuator is driven by a stepper motor. As the stepper motor turns it moves through various stop positions, and in doing so, moves the heads in and out one or more position. Each one of these positions defines a track on the surface of the disk.
Stepper motors were originally used for the actuators for hard disks as well, but were replaced by voice coils due to problems with reliability and speed. Since the stepper motor uses pre-defined track placements, thermal expansion in hard disks can cause errors in older hard disks that use stepper motor actuators, when the disk platters expand and move the tracks to a place different than where the heads are expecting them. This is not an issue for floppy disks because of their much lower track density, plus the fact that thermal expansion isn't nearly as big of an issue for floppies.
Over time, however, a floppy disk can develop difficulties if the track positioning of the actuator drifts from what is normal. This is called a head alignment problem. When the heads become misaligned, you may notice that disks will work if formatted, written and then read in the same drive, but not if moved from one drive to another. This is because the formatting of the floppy is what defines where the data is placed. Misalignment can be solved by having the heads on the floppy disk realigned. This was a common practice when floppy drives cost $500; now that a new disk costs around $30 maximum, nobody realigns regular floppy disk drives, since the realignment labor costs more than a new drive.
The head actuators on a floppy disk are very slow, compared to hard disks, which makes their seek time much higher. While a hard disk's actuator can move from the innermost to outermost tracks (full-stroke seek) in about 20 milliseconds, a floppy disk will typically take 10 times that amount of time or more. This is one reason why floppy disks are much slower than hard disks.
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