2TB Ultra killed by Mac OS upgrade?

Bought a new 27" iMac in March, and a new 2TB Ultra through Amazon at the same time. Wiped and reformatted the drive with OS X’s Disk Utility and then partitioned it in 2 halves: one for Time Machine backups and one for music/photos/docs etc so as not to fill up the iMac’s internal SSD. It worked flawlessly until yesterday when I told my iMac to go ahead and upgrade itself to 10.11.5 (from .4) and then went out to lunch.
When I returned a couple of hours later, the upgrade was done and the iMac had booted up normally – except the Ultra hadn’t mounted. The activity light on the drive was flashing slowly (idle mode, I guess), Disk Utility and Disk Warrior don’t see the drive at all, nor did listing drives in Terminal, but it does show up in the System Profiler as an attached USB 3.0 device. I’ve tried plugging the drive into a different port on the iMac, as well as through a 3.0 USB hub with the same result. I even tried plugging it into a USB 2.0 port on an old 2008 Mac Pro (also running El Cap) with the same results. When I reboot or reattach the drive the light flashes quickly as if it’s attempting to communicate/mount, but then it eventually gives up and returns to idle mode (flashing slowly).
I’m aware that it’s still under warranty, but that doesn’t help me recover the data off it – I can’t even reformat it and start over if I wanted to. Can an OS upgrade hoark an external USB drive? What could’ve happened? What should I try next? Any suggestions would be greatly appreciated, and thanks in advance for them.

Hi there,

When you did the upgrade on the mac was this with the USB drive connected to the computer? Try to connect the drive on a different computer to see if this helps.

Yes, the drive was connected to my Mac at the time of the upgrade, and as part of my troubleshooting I connected it to a USB port on an Early '08 Mac Pro and got the same result: it didn’t mount to the desktop, Disk Utility couldn’t see it, but the System Profiler saw it as a connected USB device just fine. My conclusion from this is that the iMac did something to the drive during the upgrade, but without being able to access the drive, I can’t undo whatever it was that happened. I’ve also tried using a different USB cable with no effect, so it’s not the cable… Thanks for the suggestion, though.

I am going through the same issue. I hope we get a solution for this because i also have all my work on this passport. I also think there was an update with my computer and the usb cable was also connected during this update. Have the same result that you are getting trying other computers… even a mac air and an older imac and even a windows computer…
really don’t know what to do now…
if you get any information on how to recover the information pls let me know…

Thank you very much!

Unfortunately, the drive was dead and unrecoverable. I documented everything and requested a warranty replacement from WD, who kindly sent a new drive. Luckily, I have a multi-tier backup strategy, so there weren’t too many things I wasn’t able to replace. Lesson learned: I eject all external drives (except my Time Machine drive) when upgrading the OS.