After a long search of fixes, I think the USB drive random ejection problem on my MacPro is finally fixed! The cause is the Mac's energy saving feature is not compatible with my OWC drives. OWC has implemented their own energy saving feature. I have two OWC hard drives for Time Machine backups.
The solution is very simple: uncheck the "put hard disks to sleep when possible" box in the Energy Saver under Sysystem Preferences.
The solution is very simple: uncheck the "put hard disks to sleep when possible" box in the Energy Saver under Sysystem Preferences.