I think it's maybe the other way around. 11.31 is working correctly. Anyway. pstat_getdisk is supposed to return -1 on error and return the number of instances processed. You do know you could set the index to start, and get, say, 40 disks with one call?

Read the docset for pstat here (the man pages are, um, not helpful sometimes)
http://docs.hp.com/en/1216/pstat_whitepaper.pdf