From the Blog: The next feature is one we have called “Drive Idle”… over the years, a common concern with drive pooling is 1) power consumption and 2) drive wear and tear. Every time a pool is accessed, it can require many, if not all drives in the pool to spin up. In fact, this access is often a third-party app indexing the pool as a background task, again, often requiring the entire pool to spin up. Our Drive Idle technology is designed to prevent these spin ups, or, at the very least limit them. This feature should also improve pool performance and responsiveness.
From my understanding, Drive Idle caches the location of the files and only spins up the necessary HDD for access, so other HDDs in the pool can remain spinned down. From my experiences, this feature still needs some tweaking concidering speed, but we are still in Beta, so no need for worries.