For those who want to expand the number of USB drives connected to the HD720, I can report that both the 4 bay and 8 bay Mediasonic ProBox external USB3 enclosures work:
I first loaded 4 drives with content into the ProBox 4, fired up the HD720, clicked on USB and all 4 drives were there.
Same thing on the ProBox 8 with 8 drives installed.
This was a quick test and not all 8 drives had HD content on them, but they did show up under the USB tab and all were accessible; those that did have HD video on them played just fine.
Unfortunately, the tortoise slow speed transfers remain - I just tried coping a 30+ gig file from the internal HD720 hard drive to one of the external ones in the ProBox 8 and only got 3-4 mbs.
But, again, playback of HD video from the external box was just fine.
Again, I did not spend much time with this, since all my stuff is on a Synology 1812+, but I just wanted to see if it would work and it appears that it does.