Once a hard drive goes to sleep, it'll take time to wake up and if you're using it with in Logic, that's when you'll get an error message stating this fact.
Now even though I set the Energy Saver settings as follows:
The external hard drive still goes to sleep.
So what I did was create a COMMAND file: (Notice the location of the COMMAND file)
Now when I run the COMMAND file on my external drive, it'll sleep for 60 seconds (you can adjust to your perference) then wake up, write the LS output to a TEXT file, then delete that TEXT file and go right back to sleep.
When you start the COMMAND file, don't close it, minimize it.
The reason I minimize but run in the foreground is so I can see if it's still running properly
So this is a simple way to prevent the hard drive from going to sleep.