Keep your external disks always on and responsive, with NoSleepHD
Even external disks should remain alive when you disable power save mode for their USB Hub in the Windows Device Manager and in your Power Plan settings for hard drives.
In case this configuration won’t work for you, or if you just don’t like to change your settings and prefer to have more control on when your external drives remain awake, you can use a little freeware.
NoSleepHD is a portable and safe application that prevents one or several external disks to enter sleep mode, by writing an empty text file at intervals you can decide. Usually 10 minutes are enough for preventing sleep mode, but the timing of sleep mode activation changes from drive to drive.
The program is able to auto start with Windows, even remaining hidden in the System Tray, to monitor up to 5 external hard drives. You can stop and start disk monitoring any time easily by just pressing a button. Note that the program needs to start with Administrator privileges in order to work.