Using dd on video DVDs gets permission denied after about 700MB I bought three movies on DVD on eBay. I am trying to make .iso backups onto my external hard drive. The first one I tried using Disk Utility, but it only produced a partial copy (without generating any errors) so I switched to using dd, which worked on the first movie. For both the second and third movie, dd only partially ran through before giving the following error:

dd: /dev/disk2: Permission denied
1438432+0 records in
1438432+0 records out
736477184 bytes transferred in 212.743197 secs (3461813 bytes/sec)

The number of records/bytes is similar, but not exactly the same, for both movies. I have no clue what else to try, since dd is as low-level a program I know to use. I have no idea why the error would be generated mid-copy like that. Googling for this kind of error leads to answers regarding improper filesystem permissions (I'm running sudo dd) and unmounting first (I'm also running sudo umount), and AFAIK neither would lead to dd working for the first 700MB and then getting that error.

Also, I have watched these movies all the way through without incident, so the filesystem appears to be good.
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.