how to connect 1 more HD to my computer?

you must be having 2 ports on ur board for sata hard disk ?

Heh, go look-up RAID on the internet. I got two 250gb HD's in Raid0

you can add another one cause as far as i know there are 2 joint in motherboard for sata hdd. but if you dnt have then you can also have it as an extenral hdd through usb. yes i use an external hdd as usb drive. i keep backup data in it. i just need to give it power and plug it in my usb port and it works like a 80 gb flash drive. and it is also very fast just like your hdd so i think that can also be a nie option.

I'm assuming you are talking about adding hard disk to your desktop PC and not laptop. Anyway, for desktop PC, you need to open up the side casing. Ensure you have spare SATA port on your mainboard as well as available bay for the additional hard disk.