Time Machine does not find wd passport

Occasionally, when I wake up my Mac it has lost track of the WD Passport connected for time machine.

I have clicked the "back up now" selection in time machine and get the message that it cannot find the disc. The passport is flashing in normal standby mode.

This is erratic, does not happen always.

I'm running the latest update of Yosemite. This would also happen before the latest update.