Sometime USB hubs can power down ports, I ran into that a while ago on Linux systems. Maybe something configured with "power management" is powering down the port because there is no activity to the external drive.
maybe "sysctl -a | grep usb | grep power" can give a hint or two.
% sysctl -a | grep usb | grep power hw.usb.disable_port_power: 0 hw.usb.power_timeout: 30 hw.usb.timings.extra_power_up_time: 20 hw.usb.timings.port_powerup_delay: 300
I understand and with your explanation, it helps me to understand something.ProServ nice that it was that simple. "note to self, always check physical connections, including power first"
judd I'm not 100% sure so taking a guess (grep source tree would probably help):
disable_port_power, probably a boolean, could mean "don't allow a usb port to provide power" or "don't power off ports"
power_timeout, maybe "wait this long after powering up something to go check it. If it's not ready in this time, error"
The other two are probably similar to the power_timeout, but modify behavior on an error.