To: lufcfan,
here is the exact words from network adaptor manual about HDD letters :
"The network adaptor is marked with the letters "HDD" to indicate that it can be used in conjunction with the internal hard disk drive ( for playstation2) ( sold separately).Internal hard disk drive release date to be determined."
if you check your N.A. there are IDE and power socket for HDD at back of N.A.
Here is a usefull information about ps2 HDD.
http://www.sscorpio.com/~bungiefan/psbbn_faq.html