Part of a disk drive that actually interacts with the medium and does the reading/writing (when applicable).
The specific technology depends on the disk type: some heads are purely optical (like the ones found on CDs), others are magneto-optical, others are purely magnetic - but the most common ones, found in hard disks are magneto-resistive.

Disk head size influences the width of the data tracks, which influences (along with the storage properties of the medium, like magnetic coercivity) the density of information, that's to say, the bits per square inch.

The current rush is towards smaller, lighter heads that would mean thinner tracks and less energy expended in swinging the head around. In this domain, smaller is definitely better.

The time taken to actually move the head is measured in milliseconds, which is little by human standards but ages when compared to RAM access time. This means that, in a disk system, head movements will massively influence seek time - which leads to refined strategies including disk-level buffers and head movement algorithms.

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