R10 Ryzen Edition - Additional M.2 NVMe's

I have not heard of anybody trying that. But it does sound interesting to try.

The only thing I would caution for is: The PCI-Express X4 slot 3 works at X2 lanes only. 

That will cause a performance drop and could potentially lead to issues, pending on if the drive and extender can handle linking at X2 instead of X4.

That is likely why nobody has ever tried it.

"The only thing I would caution for is: The PCI-Express X4 slot 3 works at X2 lanes only."

The upper x4 slot #2 runs at x4, which is probably why he is asking about getting  an extender to use slot 2 (x4) behind the gpu, instead of just plugging a traditional pcie add in card to slot 3 (x2).

Another option is to put the gpu on the bottom x8 slot 4, remove the 2.5" hdd caddies, and stick traditional pcie add in cards to slot 1 and slot 2, slot 3 empty.  Before doing this check to make sure the bottom x8 slot is also gen 4. Someone reported it is on the v2 R10, don't know for sure. That's how I was able to use all 4 slots. 

I got confused with this when I was looking at that spec.

Service Manual 

Setup Manual 

The service manual lists them as slot 3 and slot 2 without specifying anything about the link speeds.

The setup manual lists them as PCI-Express X4 slots (2) NOTE The PCI-Express X4 slot 3 works at X2 lanes only.

I read it as they both only support X2. My bad.