It seems that Yosemite and El Capitan often and randomly expel single external hard disks greater than 3tb mounted on USB3 boxes. If you want to be aware of this problem you can search the web in different languages. In various forums many senior and corporate users of the last generation macs are, to say the least, disgusted by the persistence of this problem for at least two years. This creates a multitude of problems for those who use new generation USB3 iMacs for working. I've personally tried different hard drives (greater than 3tb on external USB3 boxes) on different iMacs getting the same problem on all new generation iMacs with USB3, retina and Yosemite and no problem on the old type with USB2 iMacs. Particularly a portable USB3 4TB External Drive (Samsung P3 2.5 ") works perfectly and always without ever being expelled on an old iMac that still has the USB2 ports. Instead it is regularly expelled after some time on next-generation retina iMacs with Yosemite and USB3. This happens if I directly connect the disk to the new generation retina iMacs with USB3 ports or if I connect it with a powered USB3 hub. I tried to perform various formatting (both NTSC and Mac) but the problem is still occurring. I also disabled the Spotlight access to disc, energy saving etc.. but the problem persists. This problem does not exist with USB3 sticks (we have many, even 250GB with various kinds of formattings). Other friends who use the iMacs for working experienced the same thing. It's a shame that Apple does not solve the problem since that persists for a long time. I ask me if we should recourse to a Class Action to force Apple to solve the problem. it's clear that if all these problems persists with iMacs many of us will be forced to give up Apple products.