You wont be able use portable media such as an external USB drive. Link below provides information on config and setup procedures to assist you that might help.

http://www.open-e.com/library/product-presentations/

Open-E Replications
2008 - Replications supported by Open-E software, March 2009
2008 - Open-E DSS Asynchronous Data Replication over a WAN, April 2009
2009 - Open-E DSS Asynchronous Volume Replication over a LAN, April 2009
2009 - Open-E DSS Asynchronous Volume Replication over a WAN, April 2009
2008 - Open-E DSS Synchronous Volume Replication over a LAN, April 2009
2009 - Open-E DSS Synchronous Volume Replication over a WAN, April 2009
2008 - Open-E DSS Synchronous Volume Replication with Failover over a LAN, April 2009