The difference between the 2 is that if you want to access your V6 remotely using "TV Control" app, then if it's in standby it needs to be "connected" lower-power to work.
I'd like to hope that if your box is in standby, the only reason for the HDD being on would be either recording something, running an EPG update, or someone's using the TV Control app to connect to it.
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Thanks japitts - Its only me in the house - so nothing is connected to the box yet and their aren't any recordings yet - I am trying to transfer all the series links from the old box - but that doesn't seem to be working. The 'humming' is very noticeable even with the volume up higher than normal - so I'm guessing it some kind of fault. Thanks for your help anyway.