Disk Utility is failing to format an 8T external hard drive on my iMac.

I keep getting an error stating Unable to write to the last block of the device: (see below):


How might I troubleshoot this?

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1 Answer 1


There's a few reasons why this can happen, but to start with I would try two very simple things: (1) plugging the drive into a different port, and (2) trying a different cable.

If that doesn't work I would try using Disk Utility from macOS Recovery.

You don't specify the version of macOS you're running, but you should be able to use these steps to use Disk Utility from macOS Recovery

  1. Fully shutdown your iMac
  2. Power up your iMac and immediately press and hold the Command ⌘ + R keys
  3. Let go of the keys once you see the Apple logo
  4. Eventually you'll see the macOS Utilities window
  5. Now select the Disk Utility option
  6. When Disk Utility opens, use it as you normally would to try and format the external drive

If you still have no joy, post a comment below to let me know what happens.

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