Disksomnia

Disksomnia is a free utility application that stops external disks from going to sleep.

Video editing and sleeping disks are a bad mix, particularly if you have media spread across several external disks. If you’ve ever moved the playhead to another part of the sequence and stared at the spinning beachball while a disk wakes up then you'll know exactly what we mean!

In theory, unchecking the "Put the hard disk(s) to sleep when possible" option in the System Preferences > Energy Saver pane should stop disks from going to sleep. However some disks ignore this setting and will still go to sleep if they haven't been accessed for a few minutes. Disksomnia aims to solve this problem by writing a tiny invisible file every 5 minutes when Final Cut Pro or Express are running.