I have an Envoy Pro Enclosure (OWCMAU3ENPCIECS / OWCMAU3ENPRPCIU) [w/ OEM SSD - 512GB Mid-2015 Macbook Pro Retina SSD] that I use as a bootable external MacOS installation and it works fine.
I have been unable to upgrade MacOS on this external drive (I can with other enclosures/SSDs). When I commence the upgrade process, the computer restarts the 1st time and starts 'upgrading' but when it restarts again it always defaults to the internal drive and the installation doesn't complete. I've set the startup drive in System Preferences to the external drive.
I think the drive isn't seen immediately at startup and this is causing the problem. The only option I can see now is reinserting the OEM SSD into the MacBook to upgrade and then reinserting it into the enclosure - a bit of a pain.
Is there some trick to upgrading the OS on this kind of enclosure? OWC went silent when asked...