Slow Finder - Caused by external disks? I've been finding Finder slow on my brand new Retina MacBook Pro.

I've got two external disks connected to my Mac most of the time. One is a Time Machine backup, the other is a Clone, also for backup. They spin down when not in use which is great. However, I've noticed that Finder is slow to respond. When I access Finder, I hear them spin up and then it is responsive.

Is there anything I can do about this?
You can try unchecking "Put the hard disk(s) to sleep when possible" on System Preferences > Energy Saver (source).

There's also an app that says it will give more detailed control called (appropriately) "Keep Drive Spinning". 
Here is a link to the developer site, the app sends you to macupdate for the download –be careful with bundled software on the "install now" link. I have not tried it myself.