Western Digital mobile hard drives, at least 640 Gb model, have some unpleasant features on Mac OS X, regardless whether is it a root volume or an external drive. When idle, disk spins down and then spins up several times a minute. Mac OS X’s power saving settings do not matter. This behavior is not only annoying but also harmful for the device.
How to optimize Lightroom performance
You may google this topic and found several advices on a) tricking with Camera RAW cache size, and b) disabling .XMP file option. Personally I dislike disabling .XMP’s. That’s why: if a Lightroom catalog file fails you’ll stay with fresh unprocessed RAWs and all your work with developing files will be lost. (Of course you can manually export develop settings, but should you remember?)
Hence I offer another method. Idea is simple: turn off some of abundant Lightroom modules, or plug-ins, and application will start and work faster.