我電腦內有100GB data, 想back-up有何選擇?

The only practical backup solution is to buy a portable USB 2.0 external harddisk. And it will still take 1-2 hours to complete the whole 100G data. This should cost you $500 - $600, depending on the brand and capacity.

Don't try to burn DVD, not even duel layer ones. It will still be too slow and unconvenient when you have 100G data, and it is less reliable compared with USB external harddisk.