What does input/output error mean with Disk Utility

What does input/output error mean with Disk Utility

I began trying to backup my dvd collection to an external hard drive. I do this my making disk images(.iso) from the dvds. To do this I use disk utility > new image > save as dvd/cd master. Started with some Battlestar Galactica. First 6 disks of season 1 went fine. Next 3 disks of season 2 went fine. First disk of season 2.5 I get an input/output error about 75% through the process. Any ideas?