After turning on Time Machine I remembered I had old files on my external HD. So I switched off the the procedure mid way in order to first erase all the files. But that seems to have screwed things up, for although my ext. HD now shows the Time Machine Icon it is not backing up anything after several hours of supposedly backing up on the hour and reporting that it will back up on the next hour. The ext. HD is blank.
I want to return to the beginning so that when I turn on my ext. HD it will cause Time Machine to start the procedure as if for the first time. How can I do this?