It seems the first official PS2 Network Adaptor (SCPH-10190) was only sold in Japan, it was compatible with early Japan-only PS2 models (SCPH-1X000), it has a PC Card form factor, it has a connector to attach the official Japan-only HDD (SCPH-20400). The latter is external (it actually contains a drive with SCPH-20401 branding inside the external enclosure), power to it is provided using an additional external power supply and it has a capacity of 40 GB.
As for the official internal PS2 HDD (SCPH-20401), it is to be used on the official Expansion Bay Type Network Adaptors (SCPH-10250, SCPH-10281 and SCPH-10350) on any console that has the Expansion Bay (SCPH-3XXXX or SCPH-5XXXX), it comes with mounting brackets preinstalled (the bracket type depends on height of the actual OEM drive), power to it is provided by the console through the Network Adaptor and it has a capacity of 40 GB.