While this error is rare, it can be difficult to track down the actual culprit.  Typically, the error is caused by the server not having enough disk space for MySQL to perform the action.  Contact your Host or server admin and ask that they make sure there is sufficient space on the server.

You can also free up space that your own database is using by clearing out your Template revisions.  If you don’t want these revisions to be saved in the future you can disable that feature under Admin > Template Preferences.