Hard disk drives store data on a series of magnetic disks called platters. There is an actuator arm with read/write heads attached to it. This arm is used to place the read/write heads over the correct area of the drive to read or write data. Because the data is written over different locationson the disk, in order for the heads to read, the disk platters have to be constantly spinning and this creates a wait time until the desired sectors are reached. Typically it takes a hard drive 10 to 15 milliseconds to find data on the drive and begin reading it. Most of the low-end consumer HDD's have a spinning rate of 5200 RPMs while some enterprise disks reach up to 15000 RPMs.
Solid-State drives as you may assume from the name, don't rely on moving parts or spinning disks. The data is saved to floating gates transistors. These transistors retain their power even if they are not powered up. Since the SSD's have no moving parts, they can operate alot faster than the typical HDD.
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