Up until now I've been using portable external drives with my MacBook Pro but have recently purchased an iMac retina and have a couple of queries regarding external drives. Firstly, are there any advantages of using a desktop external drive with it's own AC supply over a portable drive taking power from the USB? Portable drives are so cheap now (can get 4TB for circa £100) that I don't see any advantage of desktop drives, unless there's some advantage of using mains power. Secondly, when using external drives with the MBP I always eject them before shutting down/putting to sleep, but with the iMac I'd want to leave them in permanently. Can you shut down/put to sleep without ejecting the drive, and if you can will the drives appear back on the desktop when you start it up again or would you need to unplug/re-plug in for the drive to be recognised? Lastly, is there any harm stacking the drives (whether portable or desktop) on top of each other such as heat build up, magnetic fields etc? Any help appreciated.