I struggled with a 512GB Buffalo DriveStation Mini Thunderbolt Dual-SSD Drive Raid-Enabled Solution aka SSD-WA512T from OWC. Talked to Buffalo tech-support. Buffalo's tech-support is terrible, so I returned the device the OWC. I purchased an OWC ThunderBay 4 mini RAID and 2x Samsung 850 Pro 512GB SSD's (MZ-7KE512BW). Wound up having similar issues with the new configuration, which prompted me to experiment with using different Thunderbolt ports on my Late 2013 Mac Pro. Turns out I see very different behavior, depending on what's plugged-in where. I've been poking around in Google, but so-far I haven't seen anything that explains these differences. Is the way the Thunderbolt bus is initially scanned to determine boot-devices documented anywhere?