> how can I mount a cd from an external cdrom on SCSI ID4? I tried
   > mount -t cd9660 /dev/rz4a /mnt and some other versions of source
   > device (rrz4a, rz4c,...) but I get an error message "operation not
   > supported by device"?
   
   Just a hunch, but try mounting it read-only.  Add the "-r" flag in
   there.

actually, that error is more likely that you don't have support for
cd9660 in your kernel.  check for

file-system	CD9660

in your kernel config.  most ports can also load a cs9660 LKM but i
don't think the pmax supports lkms?