DVD Ripping Problem Alias Disk 6

[philly33flyers]

Hey all. I've come across a very strange problem with my alias season 1 disk set that i'm trying to back up. The first 5 disks were recognized by my comp fine and i was able to rip them very easily. But when i put in disk 6 my comp won't even recognize that there's a disk in the drive. I've tried this on both my dru-500a that's in my desktop and the dvd-rom that's in my laptop and in both drives the disk just doesn't get recognized by the computer so i can't rip it. i tried to clean it and try again but that didn't help either. the weird thing is, when i put it into a standalone dvd player it plays flawlessly. any reason why it wouldn't even be recognized in either of my dvd drives but just fine in a standalone? any help would be appreciated. thanks.

[philly33flyers]

anyone have any idea at all?