Is it potentially dangerous, if I cant update Linux Mint?
After trying to update Linux mint, I have got following window, stating: 'Could not download all repository indexes. The repository may no longer be available or could not be contacted because of network problems. If available an older version of the failed index will be used. Otherwise the repository will be ignored. Check your network connection and ensure the repository address in the preferences is correct.'
The repository 'cdrom://Linux Mint 18 _Sarah_ - Release amd64 20160628 xenial Release' does not have a Release file.Data from such a repository can't be authenticated and is therefore potentially dangerous to use.See apt-secure(8) manpage for repository creation and user configuration details.Symlinking final file /var/lib/apt/lists/Linux%20Mint%2018%20%5fSarah%5f%20-%20Release%20amd64%2020160628_dists_xenial_contrib_binary-amd64_Packages back to /var/lib/apt/lists/partial/Linux%20Mint%2018%20%5fSarah%5f%20-%20Release%20amd64%2020160628_dists_xenial_contrib_binary-amd64_Packages.xz failed - pkgAcqIndex::StageDownloadDone <....>
Answer to the question
"cdrom://Linux Mint 18 _Sarah_ - Release amd64 20160628 xenial Release"
You still have the CD as a Repository. You can stop this working by editing your sources list and putting a # at the beginning of the line.
What else does your sources list say? Can you copy and paste it here?