Mount external HDD readOnly option
Enter in Terminal.app:
sudo asr adjust --target /dev/disk1s2 --settype "Apple_HFS"
and then try:
diskutil mount readOnly /dev/disk1s2
Mount external HDD readOnly option
Enter in Terminal.app:
sudo asr adjust --target /dev/disk1s2 --settype "Apple_HFS"
and then try:
diskutil mount readOnly /dev/disk1s2