It took 6+ weeks to get this information out of EMC/Insignia: Retrospect has a problem handling ACLs (access control lists) on Intel Macs. So I ran "sudo fsaclctl -a -d" to disable ACL support on all of the volumes on my MacBook Pro, rebooted to ensure a consistent state, and the Retrospect Client now seems to work just fine. If you have an Intel Mac and don't need ACL support (like most Mac users?), this workaround may work for you, too.