Comment 58 for bug 1242321

None of the workarounds did anything for me.

But this fixed it (for me):
1. Power off your computer
2. Plug the device in
3. Power on the computer
4. The device will now be detected and working normally. You will also be able to unplug and replug it without restarting.
5. Restart your computer with the drive mounted.
6. Unmount the drive and disconnect it.

Now your drive will work even when not connected at boot time. Steps 5 and 6 were crucial for this.

But clearly something is amiss here since now I have a new problem (only USB3.0 ports) - when I safely remove the drive or power it off, it immediately reconnects and auto-mounts.