Using dd on video DVDs gets permission denied after about 700MB
While writing this question, I remembered something I did different with the first disk and think I figured it out. For the second two disks I was using sudo umount /Volumes/DISK_NAME to unmount the DVD before running dd, which generated the permissions error, so I tried sudo umount /dev/diskX. This works, but only once per boot (it would fail again after one successful dd, until I rebooted). But can anyone explain why it worked? I thought the two invocations of umount were equivalent.