Type of computer you are using and the exact operating system
This could be a hardware problem, faulty cable or port on the computer, but more than likely it is a setting in the operating system that is putting the external hard drive to sleep and or powering down the USB port. Maybe both at the same time.
Are the external drives powered by the USB connection or do they have their own AC power supply?
It sound like you are on a Mac with limited internal drive space.
Sorry. Shows how often I ask questions and seek help. It's an HP ENVY Curved 34‑a010 with Windows 10 Home 64. The external hard drives are powered by their own AC power supply.
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Not sure exactly where you find it on Win 10 but on Win 7 there is an Advanced Power option settings dialog that give you all kinds of options.
It could also be in the software you installed for the external drive (If you did install any software for it) that is putting the drive to sleep after a period of time.
Thanks. Will give it a try.
Before being able to adjust the settings as suggested above, she tried turning off the other external hard drive with AC power which seemed to help. The power settings were subsequently adjusted as suggested above. Thanks!