Why can't I mount a Samsung SSD 850 EVO?
I have the exact same problem and to the best I can tell, it's the chipset used in the OWC Mercury Elite Pro minis. Very disappointing.

My 500GB 840 EVO works fine in these enclosures, but I've bought and returned two 850 EVOs thinking they were DOA. Suspicious after the second one, I spoke with Samsung support on 5/12/15. The rep immediately acknowledged my problem, said the Pro series does not have this problem, and that they're working on a firmware update to fix the 850 EVOs for use in external enclosures. He had no idea when that update would come though. I called back today (7/29/15) for an update, rep was mildly rude, said there will never be an update, and that our only option was Samsung's new USB3 external drives (which are 850 EVOs internally).

OWC support (7/29/15) says they do not test Samsung SSDs in any of their enclosures. Their own Mercury SSDs will work, but are not tested above the 480GB capacity.

In short, try a different enclosure.