It is a kind of a hard disk drive that is connected to the computer as an external unit and is portable. The connection of the external hard disk to the computer is made by using a cable that is compatible with smaller computer system interface. The commercial hard discs used first were too big to be kept within the PC itself. The hard disc platters were stored within the memory units which sit outside. The discs were mounted into bay inside the computer because they were transformed into a much smaller size.

The internal hard drives became first choice for computer running window by the end of 20th century while on apple Macintosh and other platform they popular for much longer. Easing up expansion external expansion easy, SCSI external hard drives were the only reasonable options for many reasons. The “Apple profile” like drives is neither portable nor compact as were the older ones for the commercial use. The need for storing files has enhanced after the introduction of the internet and the threats of viruses have allowed more backups to come. Users of all platforms consider the external hard drives popular among themselves.

In the recent times the markets of external drives are concentrating more towards the portability and additionally the memory with a higher amount. Today manufacturers like Hitachi, Simple tech, and Maxtor are using the high capacity such as 1TB and 1.5TB. There is a similarity between the external and a normal hard disc in terms of structure. A normal hard disc drive mounted on a disc enclosure is included by them.

The external hard disc drive of 500GB capacity is now manufactured on the common basis.

Windows, UNIX and the Linux are the three major systems the external drives are at ease. For each OS there are tools for formatting, journaling and partitioning. The OS can also be installed on this hard drive. Different file system can also be run on these external hard discs. Disk management console helps the partitioning and the formatting of the discs.