External Hard Drive not Mounting

Thanks so much this fixed my problem straight away!!

I tried this but the dmg file I D/L from Seagate says it's damaged so I'm still stuck.

I've tried to D/L it numerous times & even from different browsers. Always the same - the file is damaged

I have 2 6 TB WD My Book Duo connected to my 2011 iMac 27", using thunderbolt cables and ports.

One contains my Time Machine and the other ALL MY RAW and JPG PHOTO files...

Since I upgraded to Mac OX Sierra, they randomly fail to mount.

Restart does not help, changing cables does not help.

I cannot see them in Finder, Disk Utility or System Information.

Sometimes, one or both of them randomly mount, and in that case I have run a diagnostic of each drive using the WD tool app that came with the drives and they are fine. If running the diagnostic with Disk Utility, they unmount and dissapear again.

After the computer goes to sleep and Time Machine attempts to run a backup, the Time Machine drive will dissapear.

Please Apple, take care of this,...

Is this an Apple attempt to force us out of external drive and into very expensive iCloud storage?

Similar problem with the released verion of 10.12.1. External drives plugged into a USB3 hub have ceased to work after "upgrading" from El Capitan.

For hours, I was trying multiple restarts and even installing the latest update, but my hard disk was still unreadable.

Reinstalling the Seagate drivers worked like magic!

My situation:

WD My Passport Ultra 2T

I formatted it Mac OS Extended (Journaled, Encrypted) to use with Crash Plan as a back up drive. It works fine until I eject the disk and put the computer to sleep.

When I plug the drive back in before or after waking the computer, the OS does not see the drive. It is not visible in Disk Utility. I have to perform a reboot in order for the disk to mount.

Computer: iMac 27-inch, late 2013

OS: Sierra (10.12.1)

i own 2 external hard drives, (not very new) but worked fine with all other OSX, and PC< 😝 update has made them completely unusable., they dont mount, they dont apper in disk utility, or system information.

not much to my delight all responses from apple and the hard drive manufactureres are very typically ;conflicting, each blames the other, and there is yet to be a solution.

most likely now forced to buy an new expensive upto date hdd.

I am running the latest version of Sierra and I have also run into a mounting problem with my external hard drives. The problem for me (just to reiterate) is that sometimes a drive will mount and sometimes the same drive will not mount (which means show up on the desktop as it should when it mounts correctly). I do not believe that there was any operating system release that 'fixed' this issue. When I reboot my computer (2012 Mac Mini), the hard drives that previously would not mount will mount correctly. This happened just a few minutes ago. I had two hard drives that mounted and four that would not mount. After rebooting, all the drives mount and work properly.

To those who are worried about their data, I believe the data on your external drives is intact and can be retrieved as long as the disk wasn't formatted or erased using disk utility. Don't panic! Turn off the external drive, disconnect from your computer, and reboot. Hopefully this will resolve some fraction of the problems people have experienced in this thread.

I have had this problem intermittently for over a year. My machine stays on 24/7 and I think that either the hardware or software gets into a state where mounting fails. I haven't figured out a way to isolate the problem, but maybe its related to when the external drives were originally formatted. In my case, these drives are all several years old although they were correctly formatted Mac OSX journaled before they were initially used.

Facing the exact same problem. Lights are on when mouse is connected and the harddisk is connected but not being detected. Sometimes it works and sometimes it doesnt. I need to have a permanent solution for this as it is very inconvenient.

Thunderbolt drives are also affected. Unable to be recognized on boot. Only recognized after boot on internal drive, and cannot be switched to boot from the OS System Pref. Interesting they would release a build

that defeats their prized Thunderbolt/USB3 external drive ability. Have seen this before on mac 2011 and earlier, with no solution offered. Wait for the next Beta update to potentially resolve this with no feedback on 'cause'...

Suggest you and those affected revert to the prior build, which did not have this problem...

Thanks nexkulit! This solved my problem right away, I was having the same issue with my 1TB Seagate Backup Plus Portable drive not showing up in Finder and being unable to mount it although it showed in Disk Utility grayed. Downloaded the suggested utility from Seagate, rebooted and bam it's back as if nothing had happened!

Can you please try setting the following boot-arg and see if resolves your issue: sudo nvram enable-legacy-orom-behavior=1

Worked for me to release the "Option" boot menu to external drive selection. Hope this helps someone else also...

I have a similar issue with Mac Mini late 2012 running 10.12.2 Beta (16C53a) Seagate 5TB external USB 3.0 will sporadicly not mount unless the system is rebooted. This HD is connected directly to USB 3.0 on the mini not through a hub. It worked fine under 10.11 and 10.10. I have a number of other 1 & 2TB external USB 2.0-3.0 drives WD & Seagte that all mount fine. I've sent numereous feedback reports to Apple but still no sollutions.

This solved my problem

I only recently updated to 10.12.1, and have found this problem as well with my Seagate 1TB USB HDD (not SS). The drive is mountd and visible in Disk Utility, but not in Finder. At least on several occasions, re-launching finder has fixed the problem. Still haven't worked out the exact scenarios under which the problem occurs or the Finder re-launch fixes it.