I'm sure you're likely to get many answers to this one...
All of your disks shelves should be "multi-pathed", which means connected to more than one HBA. Presumably the disk in the loop or stack in question is connected to HBAs 2b and 3b. You will find that some disks are "assigned" to one adapter or the other. If you type "sysconfig -r" you should see that some disks in each raid group are on different adapters.
Richard