Thank you for your time and apologies if this has been answered before, as the forum search kept returning zero results.
I am assembling a Pi Desktop from Element 14 using a Rapsberry Pi 3 B+ and a Dogfish 120gig mSATA with the add-on board so that I can boot via usb. I was able to successfully flash the mSATA with the latest Raspbian Stretch image, and everything is connected properly on the Pi, but it will not boot to that drive. On that same token, when I replaced the sd-card and disconnected the mSATA, it would also not boot and I had to reflash the sd-card. It is to my understanding that it wasn't necessary to do any modifying as the 3 B+ is allegedly supposed to boot from USB be default if there is no sd-card present.
Thank you in advance for your help.