As mentioned in this knowledge base article:
Amongst other My Cloud devices the EX2/Ultra is unable to handle these drives when they are encrypted.
However, it’s quite hard to explain to a customer why his recently bought EX2 is unable to handle these drives when their much cheaper 5 year old My Cloud Gen1 can unlock and use these USB drives.
I checked this and can confirm that the old Gen1 My Clouds with 4.x firmware can unlock these drives while My Cloud Gen2 and EX2 with FW 2.x can NOT.
And to be honest, I don’t understand this ‘decision’ either. I know that the 2.x firmware of the later models is based on a much slimmer design with BusyBox to make them leaner and faster.
However THIS shouldn’t have been spared out in my oppinion.