Just my two cents, but I have been using 4 external drives and 3 internal drives for years and reached the terrabyte level of storage some time ago! I have worried about my externals wearing out, so I use them primarily as backup and for video files when running FCP. As a result these drives are only on and mounted when I need them. It is not at all a hassle to turn them on or off and/or unmount as needed. So, I say play it safe. Don't leave them running all the time. There is no need.