When AIX will not release /dev/cd0 device (after umount any associted filesystems), and you get busy device error on dlpar operations, use this command to kill all processes that take busy resource:
# fuser -k -x -u -c /dev/cd0
Usually in pSeries hardware configuration is connected to a scsi controller by scsi-to-ide/sas/sata or other terrible solution bridge.
To made all this operation faster you can identify pci parent adapter and remove recursively all device.
# lsdev -l cd0 -F parent
# lsdev -l scsi0 -F parent
# lsdev -l sisscsia0 -F parent
# rmdev -Rdl pci10
Then you can move the controller from HMC to another LPAR