TiVo's are one DVR I know of that will continue to buffer tuners even when put into "Standby." However, with the more recent Roamio (S5) and latest gen known as Bolt (S6), TiVo added "power saving" options to the "Standby" mode. Now, one can set the TiVo to go into the highest power saving mode that will result in NO tuner buffering when put into "Standby."
FWIW, the very first generation of Dish In-House DVR's (50X series) had the HDD always spinning, but not buffering. Dish experienced HDD failures that seemed premature for the quality of HDD they were using, and RMA's were costing Dish money, unlike the business model of TiVo. Later, Dish pushed out firmware that had the HDD spin down when put into standby, and this seemed to fix the premature HDD failures. Also, Dish got complaints about same and subsequent generations of their early DVR's that the fan was too loud from many who had put their DVR's in the bedroom. This led to the practice of Dish DVR fans spinning low at almost all but the most hot conditions. I believe that when the 721 first came out, the fan was spinning fast--meaning LOUD. I think it was some time before Dish lowered the fan speed on the 721 to keep it quiet--and fixing things so the 721 stopped doing too many things at once, which was a big demand on the processor and heating it up--to address complaints.
TiVo DVR cooling fans were always audible or even just a bit loud, but TiVo never had to provide the kind of support Dish did and does, and TiVo owners could and still do DYI to fix issues, so it was more the TiVo users concern. However, the Roamio "base" and Roamio OTA both have fans that spin to fast, meaning LOUD, and it is a common complaint because of how the airflow was designed to pass out of the TiVo. The solution is to either install your own superior fan all at your own cost and time, or just put the thing on a laptop fan and be done with it. TiVo claims that part of the reason for the latest Bolt generation of DVR's to have the strange "bent box" design is to help with keeping things cooler in the Bolt box, and I am not aware of any major complaints about Bolt fan noise as was the case with Roamio "basic" and Roamio OTA. It seems for both Dish and TiVo DVR's, airflow UNDER the box is more important than people realize, not just side and rear.
