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2013 27" iMac: Hard Drives keep unmounting, without notice, how do I find what is causing this?  



I have a 2013 27" iMac, Over the past couple of weeks my Hard Drives have been unmounting without notice, this very concerning for several reasons, mainly possible data loss.


How can I find our what is causing this to occur, so I can fix this ASAP.


As always thanks in advance for time to read and reply,



Disk Ejected

Hi Sebastian,


I haven't experienced this myself nor heard of it happening to anyone, but the 1st thing I would check is under System Preferences -> Energy Saver ... if "Put hard disks to sleep when possible" is checked I'd uncheck it.  HTH...


Old UNIX Guy

Sebastian, I have been having the same problem over the last few months.  I finally changed out USB hubs and that seemed to do the trick for me.  Don't kno wif that will help you or not.

If at least some of those volumes are on different external drives then I suspect hardware, so look for a commonality. eg. a hub, or a cable, or a port on the iMac. 

3 Answers

Depending on how these external hard drives are connected to your Mac, they could be exceeding the required power draw for the USB Bus to which they are connected, especially if there are a number of devices on that USB Bus (either externally or internally). In some rare cases you can get effective Brown Outs when multiple devices ask for peak power at once, which can cause random unmounting of drives. 


I have a similar problem with external USB drives unmounting and remounting once in a while. So far, no ill effects, but nearly all such drives are in little regular use, e.g. for backups, or iTunes (which I don’t use a lot). I too have a 2013 Mac, but it’s a MacBook Air.


I've seen both USB hubs and (bad) USB cables cause this issue. If possible, try connecting one of the more intermittent drives directly to you Mac and see how it does.