I have what appears to be the same problem with a Seagate Backup Plus 5TB:
The disconnection occurs under heavy USB I/O (TimeMachine backup), and also simultaneously disconnects my WD Passport 2TB drive.
Bluetooth devices (mouse and keyboard) are also disconnected, and the Bluetooth icon in the title bar has a line through it
The desktop usually remains accessible via desktop sharing, when this happens, and the Bluetooth module seems to be switched off, and cannot be switched back on.
I can collect relevant logs, or other information, upon request.
jrmakosky
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Minneapolis, MN, United States