Time Machine will not complete backup with WD My Passport for Mac
I am on 10.10.3 and am trying to backup up this particular machine for the first time. I had a 1TB WD My Passport for Mac that worked fine on my older Macbook Pro (which has not been updated to 10.10.3) and have been slow in getting around to backing up this new machine. I tried the 1TB after clearing space and it wasn't working. Figured it must be something with that drive, so I bought a new one (needed more space anyway) and just received a new 2TB WD My Passport for Mac. Tried the update again via Time Machine and it went through the entire process only to tell me it couldn't complete. The drive has something on it now because it doesn't have the full 2TB free and in finder the file says "in progress" at the end of the file name under backup. Please help. I need to backup this machine and now think it is Apple related and not drive related.
iMac (Retina 5K, 27-inch, Late 2014), Other OS, Using Yosemite 10.10.3