if you check your manual all 4 models should be listed on the front cover and probably inside as well as its also on my page 4 too, they are on my x70 manuals for my 770/870/970 as its the same manual for all the models in the family , so I dont think you have read the manual properly
and no the dvdr wont allow transfer of files to the usb from the hdd , its there for transferring pictures and music to the hdd from the usb , and I think you can view or listed to the same files on the usb as a source, again its all covered in the manual
the times when ppl struggle are when they try to do things not allowed on the receiver and havent checked the manual , or its expressly forbidden in the manual , which is why the manual is so thick
if you do what is stated in the manual you should be fine , its when you try to do more than is allowed when you get into difficulties