I just got one of these TB350-BTC boards going last night (thanks deepinthemines!). Unfortunately I'm having quite a lot of problems getting it to recognize any GPUs that use the standard V006 USB 3.0 risers.
I'm running ubuntu 14.04, and have 4x GTX 1060's, as well as 2x RX580 for testing - neither GPU types show up at all in lspci within linux. As far as I can tell, the motherboard is not detecting anything in those slots at all. I can't imagine all 6 risers/cables are bad, so I'm at a bit of an immediate loss.
I've not had much time to do much more debugging than cable swaps/reseating/single card testing/etc. A straight unpowered ribbon-cable 1x -> 16x riser works fine, just can't get the USB risers to work whatsoever.
Figured I'd post here to start a discussion if others are seeing this at all - otherwise I'll report any progress (or lack thereof) here if others are interested. Any suggestions appreciated!
I was also using this machine as a means to mess around with Ryzen as well in a server setting, so it wasn't meant to be the most cost effective. One thing to note with this (and any AM4 board) is that there is no iGPU available on any current CPUs, so you will need to use the card in the 16x slot. Not a big deal for most, but if you are planning on putting these in rackmount chassis and co-locating the lack of an external video connector may be problematic. Also problematic when your risers don't work at all