I hooked up a 256GB SSD in a USB 3.0 enclosure w/ 2 usb plugs (data + power) and the drive worked. One point of caution w/ SSDs - my drive died b/c of sudden loss of power (ie. turning off the car). SSDs do some garbage collection on their own so they seem to be a little sensitive to sudden power cycling. My SSD lasted all of 2 days before it went down in Germany - thankfully Crucial is replacing it. I didn't have the 2nd usb plug plugged in to the 12v -> USB port I connected to the rear seat 12v socket. If I use that 2nd plug as well in the rear seat socket, if the car turns off, the drive will still have power to do whatever it needs to do (and the Crucial tech seems to think so).
Of course, no such issues if you're using a USB stick - used a 16GB w/o any issues after the SSD went down.
2015 228i M Sport 6mt + Track Handling Package
2012 328i Sport 6mt (retired)