I read the thread you linked to. I honestly hadn't thought about any power requirements prior to this thread.
I would guess the information supplied by foodguru concerning the ability of the DVR USB port to supply any current may be incorrect. I can't see how any USB device could work without some current . That said, I did speak with techs from my cable provider and was informed that the USB ports included in many DVRs may or may not even be functional. They are provided for the purpose foodguru wants to use it for and possibly as an access port to access the DVR firmware.
As there may not be any menu item to select where the recording will be placed you couldn't select which drive to send the file to. In foodguru's case this may not matter. In my case, I wanted to be able to move files from one DVR to another. I have 3 in my home. All are leased from the cable provider. I can't do what I wanted to do.
I still don't think supplying current from an external power adapter is a good idea. I think that there is a strong chance to damge the DVR or possibly the external hard drive.
In the case linked in #5 above, the reason for the recording failure can be attributed to the hub downgrading the USB to 1.1 speeds. Even with buffer underrun technology USB 1.1 couldn't fill the buffer fast enough and the burn would stop. DVD burners can slow the burn down but once stopped, you have made a coaster.