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The answer is quite simple: A hard drive has moving parts.

In order for a hard drive to access or write data, it must be spinning (usually at 7,200 RPMs for a desktop computer, and 5400 for a laptop.)
Then, an arm must seek the track on the drive that you want to read or write to.

A hard drive only has one arm with a head attached to it. It can only read and write one track at a time.

Though the read and write access time of a drive is only a few miliseconds, there are thousands of seeks that must be performed to start a computer. And hundreds to open a file on a fragemented drive.

Drives such as Solid State have no moving parts, thus they are considerably faster.

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