Mac Pro- 2 monitor Time Machine screen redraw issue

Late 2013 Mac Pro

OSX V. 10.10.3 (which includes the supplemental install that came a week later)

30 Inch Cinema HD Display

23 Inch Cinema HD Display

When I enter Time Machine I see the attached image on my 30 inch display (my primary display). the 23 Inch does NOT go black- it just displays whatever was showing prior to entering TM. The black box appears to be the 23 Inch Display's "cover" that has been slid over onto the 30 inch display. Time Machine DOES back up regularly according to the TM pull down. You can see the cancel button obscured by the black box.