For expanded storage, you could look into iSCSI (not the best support on the mac...

mc32 on Oct 31, 2011 | parent | context | favorite | on: Apple reportedly questioning future of Mac Pro

For expanded storage, you could look into iSCSI (not the best support on the mac) CIFS or NFS supporting subsystems like those from Synology. Not sure your thruput reqs, but in most cases one of those will suffice. YMMV.