I had to install a Debian 9.3 without having access to the internet, so I downloaded the full size offline install DVD from https://cdimage.debian.org/debian-cd/current/amd64/iso-dvd. As usual, I inserted the first DVD into the DVD Drive, started the install and everything worked fine - except for the moment where it didn't ;): I needed to switch DVDs for some software - and directly afterwards, Debian came to the "Install Grub" step - and asked politly to insert DVD 1 again - which I tried to do - but could not:
For some reason, the Debian GUI Installer had locked the DVD Drive and I could not remove the DVD - could not open it. I then used the Emergency Eject, inserted DVD 1 and closed the Drive again with very mild force.
However, the Installer did not recognize the new DVD and said, that I needed to insert a DVD. Well...
To cope with that problem, just CTRL+ALT+F1 switch to a different Terminal, enter mount /dev/cdrom /cdrom and switch back to the Installer Terminal with CTRL+ALT+F7 (I think it was ;)) - and hit "Continue" - and from that moment on, it worked again and the installation ended successful after the GRUB installation.