diff --git a/os-probes/mounted/powerpc/20macosx b/os-probes/mounted/powerpc/20macosx
index 2fc7e85..ce51d63 100755
--- a/os-probes/mounted/powerpc/20macosx
+++ b/os-probes/mounted/powerpc/20macosx
@@ -23,7 +23,7 @@ esac
 # but I don't think it exists on Mac OS <= 9, and it's XML so parsing in
 # shell will be nasty.
 
-if [ -e "$2/mach_kernel" ]; then
+if [ -e "$2/mach_kernel" ] && ! dd if="$2/mach_kernel" count=1 bs=5 2>/dev/null | grep -aq Dummy ; then
   label="$(count_next_label MacOSX)"
   result "$1:Mac OS X:$label:macosx"
   exit 0