One of the issues that I have run into in the past when working with iscsi disks occurs when a host (initiator) is unable to logout gracefully from an iscsi disk when attempting to shutdown or reboot a host. Note that this is a known issue and is documented here and here. I have not read completely through the RHEL 6 Bugzilla to see if the issue has been resolved, however I can tell you that I have seen this issue in RHEL 6.2.