So, different applications all freeze whenever you attempt to write to disk? Then, we can rule out LibreOffice out of the equation...
You said that you tried to use the "Save As" option, but when did it freeze exactly: did you ever selected the location of the file or did it freeze immediately?
Are you able to create an empty document by right-click -> New Document -> LibreOffice Writer? Don't open it, just create it on the spot and see if that works. It doesn't have to be LibreOffice, just try to create any file.
After that, try to do a few ordinary tasks: Rename a file to something else, make a copy of an existing file, move a file to another folder, etc.
If you have an external hard drive, are you able to save files to it (assuming the "Save As" option allows you to pick a location)?
You may also try to run nautilus as an administrator and see if that makes a difference, in case it is a permissions issue: