外置hard-disc 好用嗎？貴唔貴？

Using external harddisk as backup is good. People said that it's better than CD. As there are cases of not good quality CDs are not readable after several years.

Toshiba and Maxtor Harddisk are good.

Assume you handle the hard disk w proper caring (keep away from water, no crash especially when u are reading and writing data), data backup by hard disk is quite reliable.

For pricing, it varies a lot from time to time. For your reference, 100GB / 5400rpm/ 2.5" USB2.0 is around $800, 500GB / 7200rpm / USB2.0 is around $2000