Using dd on video DVDs gets permission denied after about 700MB

Solution 1:

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.

Solution 2:

I've had some success. After trying a number of things unsuccessfully, including uninstalling packages that link into the filesystem, I found something that works reliably: load the disc using any software that reads the content and unlocks the encryption. Even the built-in DVD player works. Now after inserting a disc, I launch the DVD player and let the opening scene play. After that, dd seems to copy just fine.