I have lost some files since updating to High Sierra

I understand but I surmise I was not clear: if you are using Time Machine you do not need to have your external drive available all the time. Time Machine creates "local snapshots" that can be used to retrieve files you may have lost in the exact same manner as retrieving them from the external Time Machine backup drive. Read about it here: About Time Machine local snapshots - Apple Support. Restoring files, folders, or even entire systems works the same way. It just can't create as many backups, and its backup date range won't be as long as those on the external drive.