Hi,
well... in the past I had also to support those H-machines with SSA disks.
As far as I remember you have to check the hdisk-pdisk relationship. cmd=ssaxlate -l xdisk (or diag).
With rmdev: 1st remove the hdisk, 2nd remove the pdisk. If you want to identify (let blink) the disk in the drawer, first do this (ssaidentify), remove the disk-HW, then afterwards remove the pdisk entry from AIX (ODM).
hth