The iSCSI LUN components of QTS were redesigned in QTS 4.1. LUNs created in a version of QTS earlier than 4.1 cannot be imported into QTS 4.1 or later. This prevents newer NAS models from importing or restoring LUNs from legacy NAS models.
To work around this issue, back up the data on the old LUN from the iSCSI initiator server. You can also use the "remote disk" feature on the new NAS to mount the old LUN, and then copy the data to a new LUN.