If by standby you mean with the 211 off but still powered (in other words, you don't have a particular TV show on, but you're still getting the screen saver on your monitor), then that's the state my unit was in last night and the drive stayed out of sleep mode.The way the 211 accesses the EHD is completely different from the way the 622/722 do. The 211 is constantly buffering data to and from the EHD, whereas the 622/722 only read/write the EHD when you order it to play or transfer a program.
It appears that the 211 will keep the EHD awake unless it goes into standby, maybe even then.
Dish engineers might look at adding the buffering to the EHD on the 622/722 to alleviate the EHD sleep problem and maybe, just maybe enable direct writing of data to the EHD, skipping the transfer process completely. Of course this would take a little space from storage on the EHD, but it would be trivial.
I believe you are correct about the data buffering. My EHD shows a constant level of activity even when I'm not using the DVR function.