I had an issue mounting my SSD in the 800D. The problem ended up being that the caddy with the SSD in it wasn't going in perfectly level, meaning the SSD was actually missing the connectors at the back, where a normal hard disc was heavier and went in flat and worked.

Not sure if it's your issue, but certainly worth checking, as I struggled for hours with this problem. I think it's because there are some springy metal bits on the caddy which push it up.