The machine got a second hard disk, this hard disk as never shown under MorphOS. Not visible on HDConfig or on mounter. However it had MorphOS installed, so perhaps it was used as a single hard disk or it was visible under an older version of MorphOS. So yes the OS was installed there as well.

I booted MAcOSX and erased it.

I have seen this bug (?) before on PowerMacs: On machines with two hard disks only one would be recognized.