So far I'm thinking most of the external SSDs are useless because they are USB (no TRIM). I also don't really like the TB3 SSDs (e.g. Samsung X5), because they're hot and can't be upgraded. An M.2 to TB3 bridge is $170 and so it seems the best approach is to just buy a second TB3 enclosure (e.g. $200 Akitio node lite or a second Razer Core X) for the external SSD - which is what I've now done. This will keep the SSD cool and allow TRIM support. Edit: My second Razer Core X arrived today. It was sold out everywhere but Razer.com had them in stock. My external SSD is hitting full speed. Having a Mac mini with two TB3 enclosures is fairly silly, but it definitely works really well. The Samsung X5 is basically the same thing with a 970 EVO inside.