Just out of curiousity.

What purpose does the solid circle to the left of the battery charge indicator on the display screen of the Sansa Clip serve? 

That only appears if you have licensed (such as Rhapsody To Go) tracks on your Sansa. A full circle means the license is okay, a half-full circle means the license needs to be renewed within ~14 days, and an empty circle means they expire soon if not renewed. Licensed tracks that expire don’t self-delete, but they won’t play either.

Thank you.  My inquisitiveness would have driven me mad if I’d never received an answer for that.