I have a mid 2011 imac on OSX I want to upgrade to El Capitan. I bought a Toshiba Canvio 2TB external hard drive to back up before I upgraded. I get a constant stream of adress book core data errors when hooking it up to Time machine. What am I doing

I want to update my mid 2011 imac from OSX to El Capitan. I bought a Toshiba Canvio 2TB external back up drive so back up before I updated. There is a constant warning stream in Timemachine about address source...
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