The VS1680's appendices have a list of compatible IDE drives. Basically, it's the same kind of hard drive you use in your personal computer. You can get these inexpensively via eBay, tigerdirect (and other computer wholesale places), or you can r@pe a PC that no-one in your house (or work) is using.
Installation procedures are in the User Guide (it's easy: unscrew drive cover, insert carefully, re-screw cover, start up unit and format).
You might only be able to put up to a certain size drive in the 1680 (20G?, 40G?). Research that.
The Owner's Manual, User Guide, Appendices, and other VS1680 documents can be found online in PDF format by going to the Roland site, or googling "roland + vs1680 + manual" and clicking on the very first link given (at least that's how I did it).
Cheers.
My very first post!