Three external disks are connected to my desktop Mac. Even when I set System Preferences > Energy Saver to not put the disks to sleep, sometimes they sleep anyway. Here’s how to solve that.
When Sleep Is Bad
It’s not that a sleeping Mac disk is bad, but sometimes, when external disks are asleep, opening a file takes much longer as the OS X pop up dialog box needs to awaken the disk.
I don’t think there’s much difference in wear and tear between sleeping disks and running disks, so I want mine running all the time.
Even with the System Preferences settings set, sometimes those disks just go to sleep. Keep Drive Spinning is the free Mac app that, well, keeps them spinning.
If a connected disk drive is going to ignore the System Preference settings then all you need to do is run Keep Drive Spinning for each drive.
The app creates an OS X launch agent which tells OS X to check on a hidden file on the selected drive every minute or two. That extra touch keeps the disk drive awake and spinning.
Keep Drive Spinning needs to run for each connected disk drive. It uses Automator to create a launch agent which does the deed (launch agents are stored in the Launch Agents folder in the user Library).
That’s it. The launch agents run automatically in the background, just often enough that connected disk drives don’t fall asleep. That keeps them running, and won’t slow down Finder access or pop up dialog boxes.