Hard Disk Drives are nonvolatile (meaning they don’t lose data when power off) devices that store data, generally 3.5”. Types of HDD include:

SATA : Serial ATA drives come in 150, 300, and 600 MB/s versions (1.5/3.0/6.0 Gb/s), use a 15-pin power connector and a 7-pin data connector.
PATA : Parallel ATA drives range between 33 MHz and 133 MHz (Ultra ATA/33 through /133), use a 4-pin Molex power connector, 40-pin IDE ribbon cable for data, and can be jumpered as a single, master, slave or cable select.
SCSI : Small Computer System Interface drives range in transfer rates from 160 MB/s to 640 MB/s and use 68-pin, 80-pin or serial connectors.

**Source by wikipedia**

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