Originally Posted by furtsiv
if you know to solder then buy a slim scph-700xx(v12 or v13) and remove dvd player to make space to put a 2.5" hdd (you can find diagrams just google)
I would not recommend removing the drive entirely, considering the HDD compatibility is not 100%.
But yes, you can either use a fat PS2 with HDD and network adaptor, or a Slim PS2 with a HDD controller chip soldered on (there's a product called HD PRO which is worth looking into).
As mentioned before, there's an USB loader but since PS2 uses USB 1.1 the speed is lower than with DVD.
Lastly there is SMB (ethernet) which is faster than USB but still slightly slower than DVD (enough to get stuttering cutscenes).
PS3 Slim CECH-3004A 160GB (500GB)
PS2 Slim SCPH-70004 - FMCB 1.8b