The configuration of the MediaDock determines whether external drives may be attached to it.* " iS ready" MediaDocks in single bus configuration, containing at least one iS drive, do not support the attachment of external drives.* " iS ready" MediaDocks in single bus configurations, that don't contain iS drives, support the attachment of up to three external drives.* " iS ready" MediaDocks in dual bus configurations support the attachment of up to four external drives to each bus.* A non " iS ready" MediaDock in a single bus configuration supports the attachment of up to three external drives. (You are still limited to a total of seven drives.)