A while ago I e-mailed WD support asking them to check out their My Book 6tb HDD in conjunction with the WDTV Live.
They’ve now replied saying they’ve tested it with both the old WDTV Live and this new model, and it works fine.
So, if you’re happy to use both front/side and back USB ports you can attach a massive 12tb to either model. I’ve asked them if using a powered USB hub will be a problem over a certain number of HDDs, or over a certain capacity. I’d rather have 2 x 6tbs attached to a hub round the back than have one sticking out of the front.