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Definition
A Hard Disk Drive (HDD) is a non-volatile means of storing data (digitally) on a magnetic surface. Hard drives are made up of
spinning platters (usually made of aluminum or glass) that contain a very thin magnetic coating. A magnetic "head" glides over the platters and polarizes the material in microscopic sections. The head is capable of both reading the magnetic material and altering it; a.k.a. it's capable of reading and writing.
Form Factors
Early hard drives were extremely large, very heavy, and had very limited capacity. The IBM 350 RAMAC HDD (as seen on right) carried a maximum capacity of 5 megabytes! That drive was introduced in 1955.
Several larger hard drives were manufactured and introduced between the mid 1950's and 1973. In that time frame, hard drives did shrink a bit, but capacity wasn't really increased. The true "ancestor" of the modern hard drive comes from the IBM Winchester drive, invented in 1973 (pictured below and left). The Winchester was the first drive to put the platters and heads into a sealed rectangular case
. It's maximum capacity was 60MB, which for 1973 was a collosal amount of storage.