Debian :: Let Apt-get Update Ignore Installation Media?

Short version: Why does apt-get update since recently ask me for my installation DVD?

Long version: In my sources.lst there is still an entry for the installation DVD.

I like that because if a package on DVD is still the newest version then there is no need to download it again. (If I comment out the DVD entry apt-get update will ignore the DVD but at the same time no package will ever be installed from it again, right?)

Since some time ago this was no problem as apt-get update didn't have a problem with the entry - or if it had (I don't really remember), there was just an error message and the process would continue with the other sources. But then this changed and now I'm asked to insert the disk (luckily I have only one DVD) every time and the routine waits until I have done so.

Is there a way to change that back?

I've read some apt documentation and the apt-* manual pages including apt.conf and some examples for that but could not find something useful though there might be something I missed because I didn't understand everything I saw.

Clearly, it makes sense that DVDs and the like can be included in apt-get update because some people update offline with these media. But (the standard) DVDs are read-only and the individual disks are identified by apt (see /var/lib/apt/cdroms.list). So the system should not expect an already known DVD to change but recognize if a new one is being used.