Looking for a viable solution I came across the mumbi USB fan. Important for me was the arrival off switch, which is located behind the fan.
It works fine and I get my hard drives by more than 10 degrees cooled down. The volume of the fan I do not feel particularly disturbing. You can hear him, but not particularly loud.
I have now bought 3 pieces of it, so I can cool all my docks.
From me, saying an absolute recommendation for the product. Price and performance are here and also the shipping was very fast and the product was packaged very well to me.