There are several interface standards for passing data between a hard drive and a computer. The most common are IDE and SCSI. Although removable disks encased in cartridges use the same "hard" disk media and a similar drive technology, they are mostly called "removable drives" rather than hard drives. 

What does IDE and SCSI mean?
IDE (Integrated Drive Electronics) a hardware interface widely used to connect hard disks, optical disks and tape drives to a PC. Introduced in 1986 with 20MB of storage, capacities increased a thousand fold in less than two decades. Compared to the SCSI interface, IDE has been the more economical choice. IDE is a standard electronic interface used between a computer motherboard's data paths or bus and the personal computers hard drive.

SCSI was developed as an industry-wide standard. It is used on PC Servers, and was used on Macintosh, but it was also designed to handle the heavy load of performance critical central servers that cost hundreds of thousands of dollars. It is better than ATA, but more expensive. SCSI is a parallel interface standard used by motherboard's data paths or bus and the personal computers hard drive. SCSI interfaces provide for faster data transmission rates (up to 80 megabytes per second) than standard serial and parallel ports which is why SCSI hard drives are generally more expensive than IDE hard drives.

Hard Drives comes in a range of different prices. You can find many discount in hard drives from as little as Rs.800 to over Rs.4,00000. Price ranges for hard drives general depend on the manufacturer and the amount of gigabytes offered with the hard drive. There are many quality manufacturers of hard drives, like Toshiba, Western Digital, Quantum, Maxtor, Seagate, Maxtor, Hatachi, Fujitsu, and many others.

What does Gigabyte mean?
A gigabyte (derived from the SI prefix giga-) is a unit of information or computer storage equal to one billion (that is, a thousand million) bytes. One gigabyte is roughly equal to 18 hours of MP3 music (at 128 kbit/s). One gigabyte is roughly equivalent to 11 hours, 40 minutes of Flash video (at 450x370). A gigabyte is a measure of computer data storage capacity and is "roughly" a billion bytes. Today's hard drives are much less expensive than they have ever have been. Buying a 100 GB is very affordable today but as earlier as 6 years ago it was unheard of.

Hard drives internal and external.
In internal hard drive or external hard drive no matter the type of computer you have. An internal hard drive must be properly installed within the housing of the computer. To properly install an external hard drive your computer must connect with a 6-pin or 4-pin FireWire port to transfer the data between the motherboard.