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I'm trying to format my USB so I can get all the space back onto it, but every time I try to erase it in Disk Utility, this comes up:

POSIX reports: The operation couldn’t be completed. Permission denied

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  • Are you erasing the disk or the main partition? – Harcker Nov 6 '17 at 9:50
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Try with diskutil:

diskutil eraseVolume HFS+ "new name" disk1

Replace new name with the name you wish the disk to have, and disk1 with the disk identifier (select disk → Info).

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  • tried this method, but got Error: -69888: Couldn't unmount disk – Micah Stubbs Feb 25 '17 at 4:29
  • Try force unmounting by entering diskutil force unmount disk1 in terminal. – user244378 Nov 6 '17 at 9:28
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Although it may seem unintuitive, try formatting the drive using a Windows computer. Then, take the now (Windows) formatted drive over to the Mac and format it there - it will work.

It's has happened to me many times too, but the speed of this method has always greatly outweighed any further research time.

I hope that helps you out, however, the question of what causes this error could be one of many... hence, if you need a formatted drive now, this will work.
But do let me know how you get on.

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