External USB drive keeps disconnecting randomly
After studying my problem further, I identified the Newertech Ministack 2012 as the cause of my problem. 

Our drives disconnected, because they overheated. Practically, while fans start spinning when I turn the Ministack on, they do not resume spinning when drives return from standby/idle and stay off. This caused already a couple of dead hdds in our setting and other drives experienced temperatures around 70C.

Strangely the enclosures are quite clean inside, this could mean that the fan behavior has been consistently this way since we bought them. The enclosures have been bought through official channels between 2014 and 2015.

I am starting to believe this might be a production related or architectural related problem this model has. The day after I contacted Newertech the enclosures where unavailable for purchase. Usually I had good experiences with the company, but data storage is very important so I am trying to get to the cause of this issue. 

I tested different settings by turning the enclosure on, accessing the drive, letting the drive idle (usually 10 min) and go into standby, reaccessing the drive and inspecting fan behavior. I switched HDD vendors, cable, interfaces, computers, OS-X versions, this leaves the enclosure.

Could someone please give me a feedback? I would like to figure out if my enclosures just all went bad with the same problem or if we all have a very huge problem! My drives heated up to 70C and should be around 40C, with 60C the maximum advised temperature from the drive vendor.