[XCSSA] Memory Card Mod for PS Slims?

[X-otic Computer Systems of San Antonio]

Yo Peeps!

A friend of mine has another friend who has a PS2 Slim that wants to  
be able to use something like HD Advance. I remember a few meetings  
back this was done but I can't quite remember the steps nor if it  
works on PS2 Slims? I know you'd need to use a USB drive and it would  
be slow but I think the goal was to be able to play games off an HD  
for that convenience (not so much for faster load times).

Thanks!

Tim S.