Thanks for posting and for your question!
It seems that the 20 minutes standby is hardwired in the code. You wouldn’t be able to decrease the time, but you could increase it.
A possible way to do this would be to use a 3rd party program such as NoSleepHD (Open Source, so it’s free).
Unfortunately we haven’t tried it yet, so we cannot safely recommend it. If this isn’t urgent give us a couple of days and we’ll investigate this further and test it. (Got plenty of testing HDDs in the office here).
Hope this helps!