Im not to sure, I know that BSD has been running on the PS2 for sometime now, you could try searching for something like "ps2 linux bsd" or sumit,

I would have thought if a linux distro has been released for the PS2 that it would have HD support if theres a port for them
There is a PS2 Linux Distrobution, which leads me to believe Sony released a hard drive, as catch said. And yes, there is a NetBSD port to the PS2, but I believe you need the PS2 Linux kit first for the hard drive, and keyboard and everything.

Playstation 2 Linux kit/distrobution
NetBSd port (requires the PS2 Linux kit
I'm sure there are other ports of OS' avaliable too.