How to back up my imac to a WD my passport 1tb.?
I keep getting "Timemachine cannot back up message
That's a portable port powered drive and not advised to be used as a TimeMachine backup.
Use a self powered drive instead.
Use the port powered drive for portable storage like it was intended.
Apple is now cutting off the power to some port powered drives because they are drawing too much current and killing people's batteries on laptops.
Also read here for diversifing your backups, with TimeMachine people make mistakes in judgement about it and lose their data.