I too am renting a TG1682 but it is A/B/D/G/N A/C capable. I have had it for two weeks and have had a number of issues. However, the one issue I did not have is using a WD MyCloud 6TB Drive as my network BU using the USB port. I read the three responses you have received, but I have also done some reading on the COX.COM/mywifi site and have read that the USB port, while not currently enabled, will be used for an external drive.
So, at this point, I am a little confused as to why my WD MyCloud works but I am having a number of other issues. I have even had COX come out and replace the TG1682 they had installed 10 days earlier. I am now getting 50Gps DS and 5Gps US. I have 15 devices in this house connected to it and it handles them all.
I wouldn't give up on your EHD. Might want to go the the MFR Web for your drive and see if they can provide a suggestion or two. Good Luck.