For the past 3 or so weeks have been having issues with external Hard Disk unmounting/powering off. My current home server setup is : Mac Mini ( late 2014 ) 2.6GHz 16GB 1TB Fusion Drive. OS X Yosemite 10.10.4 and OS X Server 4.1.3 2 x WD My Book Studio 4TB external Hard Disks. USB3 ( 1st Hard Disk, Partition 2 TB – Home Folders and 2 TB Time Machine ) ( 2nd Hard Disk, Partition 2.5 TB – Media Files for Plex, and 1.5TB Caching ) Have been running this setup for about 6 months now, running it mostly 24/7, with a weekly or sometime twice weekly shutdown and restart. But in the last 3 weeks or so the WD My Book Studio’s USB3 have been unmounting/powering off and the only way to get it back was to unplug the power cable from hard disk casing and plug back in. So thought it was WD issue. After getting replacement power cables and now a replacement Hard Disk it is still occurring. So left pondering what to do next. Really annoying when you try to access Plex from a TV or the server from rMBP and the disk aren't there. The settings haven’t changed. The Energy saving in system preferences is set at turn off display 10mins, Put hard disks to sleep when possible, Wake for network access and Enable Power Nap. The WD Drive Utilities settings for the sleep timer are set to 10 minutes. Couldn’t tell you if it started to happen when I updated to 10.10.4 ?? Has anyone experience or has any suggestions ??