I believe what's happening is that when there's been no activity for a time, on the USB port the device is plugged into, and then that port gets selectively suspended, Windows loses the connection to the external drive. Which explains why you have to reboot the device in order for disk management to see it again. Try this: Open "Power Options" in Control Panel, click "Change plan settings", click "Change advanced power settings", scroll down to "USB settings" and click both + signs, change "USB selective suspend setting" to disabled. Close out the dialog boxes and restart the PC. Your external drive shouldn't get (selectively) disconnected now.
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