I’m trying to install the latest version of Lubuntu on my old MacBook 4,1 using a usb stick. Live Lubuntu starts fine, but the installation always stop with the following error at around 80% at Termination step:
apt-get update failing for cd-rom source saying that the release file doesn’t exist.
I tried to disable the cd-rom source using the UI Software Sources, also I tried to comment the corresponding lines in /etc/apt/sources.list
The CD-ROM refers to your installation media, and if you’ve not installed it yet and are having issues; it means, at least as I read it (providing the exact error message allows an exact search) that you’ve issues with your ISO or more commonly (in my experience) write of ISO to media.
I’d normally open a terminal, and scan the systemd journal (journalctl) and check the “casper-md5check[9999]: Check finished: no errors found” message was present. Next I’d check logs to ensure nosquashfs errors were detected; as those are read issues on media which maybe cause for your problem, and more importantly a warning that any install may not be trustworthy even if completed.
I’ll provide links to my thoughts on askubuntu on verification & media checks