
Hello,
today I will try to explain what is the Hard Disk Drive (HDD)
A hard disk drive (HDD), commonly referred to as a hard drive or hard disk
Capacity of a hard disk drive is usually quoted in gigabytes ( 1GB= 1024MB) and terabytes (1TB=1024GB). Older HDDs quoted their smaller Capacities in megabytes, some of the first drives for PCs being just 5 or 10 MB.
But today we got up to 1 terabytes HDDs ( 1TB = 1024GB )
Hard disk drives used to be massive size. but with the technology they became so small and it has more capacity

New HDD and old HDD
Hard Disk’s comes in models and speeds
HDD Models:
Currently most used is ATA and SATA.
ATA ( AT Attachment ) is the current standard, it uses regular molex power connectors and IDE (Integrated Drive Electronics) cables.
SATA ( Serial Advanced Technology Attachment ) is a newer product (also SATA2) – it is faster than ATA and uses different cables – much thinner and neater.

IDE Cabel SATA Cabel
HDD Speed:
Also take into consideration the Round Per Min (RPM) of the drive, 5400 is a bit slow, 7200 is good, 10,000 is fast but can be quite noisy.
Servers generally use 10k rpm SCSI hard drives, but SCSI is unusual in home computers.
Today many people use Extrenal HDD’s for their Data needs.

An external hard disk drive is a type of hard disk drive which is externally connected to a computer. Modern entries into the market consist of standard SATA, IDE, or SCSI hard drives in portable disk enclosures with SCSI, USB, IEEE 1394 Firewire, eSATA client interfaces to connect to the host computer.




I hope that you learn something new about this
And I guess we explained the basic information about the HDD.
Source:
Wikipedia – http://en.wikipedia.org