It's kind've a long story.
I originally had a Seagate 3TB HD which worked from the rear port. I returned it for a WD 3TB. I think there was 1 or 2 FW updates since I returned the Seagate. I noticed one day that I was having issues with the rear port. I even put in an RMA, and used the front port for a while.
For some reason the rear port started working again so I'm using that.
There are others on here that aren't as fortunate as myself though and the rear port gives them grief. But there's also quite a few people that do have 2/3TB HD's working flawlessly on both ports.
So roll the dice . . . You can always return it if you notice something off with the latest FW.
Personally I have no interest in buying another WD media player again, but for $80-$100 It's difficult to match/beat.
Why don't you use the 'search' box - You can find a lot of posts regarding the rear USB port and HD's. I have written detailed information regarding the issue that I'm not going to re-tread here.