Conventionally a "disc" is optical and a "disk" is magnetic (or equivalent)
Yep, for FHDB you only need a network adapter (very preferably a Sony original), a disk you can plug into it (oh, and remember: always turn off the mechanical power switch before inserting or removing it), a compatible USB drive (must be MBR and FAT16/32, compatibility isn't very high though reasonable and light years better than Xbox), and some way of writing to the two previous items
And, if that's got you confused, the FMCB installer is used for installing either FMCB for PS2s, FHDB for PS2s, and FMCB for PSXes
(Not really for you, but since others may read this in the future: while the 700xx models have a full network adapter onboard, and support a proper "internal" HDD, it's not bootable so it can't use FHDB directly)