In make_hfs.sh, explicitly mount the new volume.

chrome/test/data/safe_browsing/dmg/make_hfs.sh

 #!/bin/sh
 
 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This script is used to generate an HFS file system with several types of
 # files of different sizes.
 
 set -eu
@@ skipping 15 matching lines @@
   exit 1
 fi
 
 if [[ ! $OUT_FILE ]]; then
   echo "Need to specify a destination filename."
   exit 1
 fi
 
 RAMDISK_VOLUME=$(hdiutil attach -nomount ram://$RAMDISK_SIZE)
 diskutil erasevolume "${FILESYSTEM_TYPE}" "${VOLUME_NAME}" ${RAMDISK_VOLUME}
+diskutil mount ${RAMDISK_VOLUME}
 
 pushd "/Volumes/${VOLUME_NAME}"
 
 touch .metadata_never_index
 
 mkdir -p first/second/third/fourth/fifth
 
 pushd first
 pushd second
 pushd third
@@ skipping 24 matching lines @@
 
 echo "This is a test HFS+ filesystem generated by" \
     "chrome/test/data/safe_browsing/dmg/make_hfs.sh." > README.txt
 
 popd  # Original PWD
 
 # Unmount the volume, copy the raw device to a file, and then destroy it.
 diskutil unmount force ${RAMDISK_VOLUME}
 dd if=${RAMDISK_VOLUME} of="${OUT_FILE}" &> /dev/null
 diskutil eject ${RAMDISK_VOLUME}