I have been trying to upgrade the firmware to v6 on our old PS4000 series equallogic array for some time now without much success. It seems I can't go from 4.3.8 to any version past 5.1.2.
I thought I would post the upgrade path which I found through trial and error.
4.3.8 > 5.0.7 > 5.2.5 > 6.0.1