Hello, This is a bug report. I tried to use the reporting system, but it did not seem to be appropriate. So, I will give a narrative:
This is what happened. I decided to install Debian on my small machine (Asus eeePC 900A, 1GB ram, 4GB storage). So I selected debian-9.3.0-i386-xfce-CD-1.iso and put it onto a flash memory stick. Installation went ok, but later on when I did "apt install gcc", the installation program requested the insertion of the installation disk in the drive /media/cdrom. Why? Examination. The /etc/apt/sources.list file had "deb: cdrom:[<installation>..." as the first entry, and was not commented out. My installation medium was wrong! Solution. Comment out "deb: cdrom:[<installation>..." in sources.list. Discussion. There are 2 problems: (1) Installation should not put the installation medium in the sources.list, at least not without asking the user, since the medium may be reused. (2) Installation should keep track of the installation medium; if necessary by asking the user. (2) reflects a wider problem: Debian has a legacy of assuming CD or DVD installation. But much is now done by flash memory sticks. Debian should change to reflect this. For example, the iso file names might be "large" (or "full") or "small" (or "base"), not "DVD" or "CD". References to "CD" or "DVD" should be replaced by reference to the "installation medium", unless "CD" is actually necessary. Etc. Regards, John Hosack