I have two WD my Passport for Mac (3TB, 3.0 USB) portable HDDs permanently connected direct to my iMac 27" 2015 Retina (not via any USB hub). I use one as TimeMachine and one as storage for photographs. I permanently leave my iMac turned on. After a couple of days (intermittent time frame), the contents of the two HDDs disappear. The drives still show on my desktop and in finder, but the contents are not there. The power lights still flash on the HDDs as if working too. This also causes the TimeMachine backup to fail. Things I have already tried: 1) in the WD software, turned off the power save option 2) removed the drives from Spotlight search I still have the same problem though. Does anyone have any ideas on what else I can try? many thanks!