http://h20000.www2.hp.com/bc/docs/support/SupportManual/c00378986/c00378986.pdf?jumpid=reg_R1002_USEN might help. There is a fairly recently method of remirroring using the ACU booted from a CD containing the ACU - probably the SmartStart CD under maintain server.
Before this method it was sometimes impossible to re-establish a mirror because automatic rebuild requires the correct slots (which curiously should work in your case anyway) which was sometimes impossible if a SCSI ID changed through a backplane problem>