D.D. said:the official sony one does this. some modchips (dunno if i shud name them all but i will say that two of the most popular ones starts with an M and ends with an H and the other one starts with a D and ends with a Z and has only 3 letters, followed by an integer that equals the number of alpha letters in its name >.< ) allow this with ANY harddrive thru ATAP patching. this also enables games that use the HD (official) to cache, download maps, etc onto the HD (ie socom). These modchips also offer the capability of HDloader/Advance built in.
You know that is a pretty big hint and the idea of being subtle just went out the door hehe. But yes, it is great having one and using it for more than just playing PS2 games. Another benefit these advance chips offer is the ability to make the PS2 act as an FTP server, and so I am able to have a secondary backup of my mem card saves to the PC as well as the official Sony HDD (you never know what could happen).