I use Macs - primarily a late 2013 iMac 27" running OS X 10.9.4, and an HP Elite laptop with Windows 8.1 and USB 3.0. Some users say that it works fine on Windows and not so great on Mac, but I see the same issues on both platforms.
I installed 4 drives: 1TB WD Green 500GB WD Green Samsung SpinPoint 500GB Hitachi DeskStar 500GB
Not connected to anything, with just one drive installed, it will spin down that drive, and turn off the fan after about 30 seconds to a minute. Install more than one drive, and all the drives spin down, then spin up again after a few seconds, while the fan stops. You can clearly hear the drives whirring... So you basically can't have the enclosure turned on and hope for it to sleep when not used, because your drives will keep spinning, but now without cooling. It does not matter which of these drives are installed - the WD drives are Green models, and is practically made for spindown support. Any drive, when installed alone, will spin down in this enclosure. More than one, doesn't work.
Computer Sleep Issues: When connected directly to a USB 3.0 port on either the HP laptop, on the iMac, or on a USB 2.0 port on my MacBook Pro, sleep does not work either: On the HP, when the computer sleeps, the enclosure will stop the fan, but keep the drives spinning. When the HP shuts down, the same thing happens. On both Macs, when entering sleep-mode, the enclosure does not enter sleep-mode ever. Even with "shut down drives when not in use" are enabled, no drives in this enclosure ever spin down. When shutting down both Macs, the same thing as with the HP above happens, the enclosure think it's sleeping, all the LEDs turn off, but all drives keep spinning.
It's really a shame - this is a super nice enclosure, well built and sturdy, nice cooling, but unfortunately unusable. Others may not see these issues...