Describe one type of secondary device in detail

Mobile hard disk is a kind of secondary devices , because of its high-capacity and portability , it is be used more and more widely . It has some advantages : firstly , it provides a high-capacity for users in a acceptable price . The most mobile hard disk on the market provide capacities range from 300GB to 4TB , several even up to 12TB . Secondly , mobile hard disks are generally small , and they are very convenient to carry-on . Then , mobile hard disks are fast writing and reading , they most use USB , IEEE1394 or eSATA interface , so they can be fast to exchange data with CPU . But to some extent , the speed of exchanging data is limited by the transmission speed of the interfaces , so when users are transmitting large files they need to be patient . Finally , the reliability of mobile hard disks is good , their structure resembles that of computer’s hard disks . They are usually made of oxygen silicon disk which is more sturdy and durable than aluminum and magnetism , so they improve the reliability of data well .

With the development of hard disks , the mobile hard disk will be more small , portable and high-capacity .