Is the HD 2.5 or 3.5 inch? Is it possible to install say a 120Gb 5400rpm Maxtor?

It's a 3.5 inch (since this is a home unit).

Exactly which drives you can install, I don't know about. I would guess that just about anything would work (like the car player), but I'm not sure. Empeg guys, any comment on this?

Can I *REALLY* use this as my MP3 server? i.e. Can I export it's music database to a LAN via NFS, SMB etc?

Currently it doesn't have NFS or Samba on it, I don't think. It's designed to serve other Rio products, so its built-in protocols are proprietary.

HOWEVER...

Those proprietary protocols are well-documented (they were reverse-engineered for the Rio Receiver) so you could write your own receiver application,

AND...

Just like the car player, it runs a pretty straightforward Debian subset, so adding Samba shouldn't be too tough.

OR...

You could probably get Displayserver running on the thing with little trouble and it could be an HTTP music server.