This usually happens during head swap, when new head comes as full filer package with own disks. Then if you simply reassign old filer disks, you end up with two diskroot's. Which one is then selected for booting? Is it random? Or are there any rules for it?
For models with external shelves I always connect new shelves only after head swap is finished. But for the case of integrated disks it is not that easy, and requires more fuss around. The less manual work, the more reliable is procedure
