For the original PS2 hard drive you'll need HDAdvance (how to use it: wow.hdadvance.com/hd_advance_how-to-use.htm). For the slim line PS2 you'll need a USB hard disk and USB Extreme (how to use it: wow.hdadvance.com/usb_extreme_how-to-use.htm).
I wouldn't recommend buying the above from ebay. As the auctions quite often get closed, because these programs can be used for playing pirated software.
(Disclaimer: I, nor Digital World for that matter, condone the use of pirated software and/or games. The above info is for education only, and is given to help those people who wish to back up their legally owned PS2 games onto a HDD to save risk of damage to the original disk and wearing out the eject button of their PlayStation).