My Book Live 3TB speed issues with Time Machine

After buying My Book Live 3TB to use as Time Machine for 1TB iMac and 1.5TB external drive I tried to do the initial back up wirelessly as advised with the instructions. After 16 days it hjad still only backed up 2TB of the 2.37TB 

I am currently trying to back up with direct ethernet connection to the iMac. My Book Live backed up the first 1TB in a matter of hours but has since taken more than two days to get from 1TB progress to 1.6TB progress so at this rate the back up will take almost another three days to complete

I connected as Guest because My Book Live would not accept my admin name and password combination. Is this why it is taking so long, or what else do I need to do? Is there a bug in the firmware and when will it be fixed?