I've discovered the difference. For whatever reason, the 6.0 version of flexbackup uses the "tar" method for the backup while 5.6 uses the "dump" method. To access 5.6 tapes on a 6.0 server one must change the method to "dump". To do that, edit the template fragment 10type in /etc/e-smith/templates/etc/flexbackup.conf. Simply change "tar" to "dump". I expanded the template by configuring tape options from the server manager but I'm sure there's a way to do it from the command line as well.
I'd like to thank Brian Read for his help on this. My query here went unanswered but Brian pointed me in the right direction.
