Try this.
1.Go to your Control Panel > System > Hardware tab > and click on the "Device Manager" button.
2.In the Device Manager window, find the item or items labeled "Universal Serial Bus controllers." Open it, and double-click "USB Root Hub."
3.In the USB Root Hub Properties dialog, click on the "Power Management" tab.
4."Allow the computer to turn off this device to save power" is checked by default. Uncheck this box, and click "OK."
There may well be more than one USB Root Hub.
You mention; "usually", and "sometimes", in your post, which could point to a dodgy cable/connection snag. Does moving the cable when connected have an effect?
Power supply might also be a player, insofar as the external drive needs a bit more oomph than the computer can supply.