Hello friends!
About 3 years ago, the same problems began with the disappearance of external drives in other operating systems on HighPoint SSD7101A-1, Sonnet Fusion Flex J3 i, frequent reboots, turning off/on the computer was very annoying.
Fortunately, there were also pleasant moments when the external system worked properly for weeks. I began to notice a pattern and am sharing my experience with you, I hope it will help someone. I have been working steadily for about a year.
If you need to work on an external macOS drive, then when you turn on the computer, you must first boot into the system on the main Apple drive and reboot through “System Settings - General - Boot Disk” to another drive with macOS or Windows, Bootcamp.
When you turn on MacPro 7.1, do not boot the system from external media through the Option/Alt boot disk selection menu, this method is only applicable for the Apple disk (after updating the system there may be failures, but the Option/Alt key when turning on my computer helps boot into the main system Macintosh HD).
For clarity, I will attach a screenshot.
Sincerely, Natalia.
View attachment 2394103