Keep Drive Spinning ensures that a connected drive never goes into low-power mode.
Keep Drive Spinning ensures that a connected drive never goes into low-power mode. Normally this can be controlled using the Energy Saver preference pane in System Preferences, and that should be your first resort in dealing with the problem, rather than this or any other third party software.
This applet simply creates a launch agent that tells OS X to update a hidden file on the selected drive once every minute so that the drive stays awake. Launch agents are built-in OS X technology and persist across reboots, etc., so you can run this once to set-it-and-forget-it. You can find the launch agents made by Keep Drive Spinning in the LaunchAgents folder of your user's Library folder.
Users on OS X 10.4 (Tiger) and OS X 10.5 (Leopard) need to run the AppleScript source code directly. This source code is included, and only requires opening the file and then clicking the big, friendly Run button.
I had the exact same issue. I installed version 1.3 which offered to try with admin privileges and that did the trick! Works now! Mac OS X Yosemite.
This app seemed to do the trick the first day I had it in use with an external WD 3TB USB3 drive, but this morning the drive was asleep with no settings changed in 10.8.5 so something is still being overridden at some point which defeats the point of the app.