I have to always format external disks twice on my laptop (Macbook Pro, macOS Sierra). The first time I try to format an external disk it always fails. This happens with any disk, any format type, and regardless of whether I'm using Disk Utility or Terminal.

I would like to know what might cause this issue?

  • Details? Any disk, any format type?
    – Tetsujin
    May 5, 2017 at 7:02
  • @Tetsujin yes, any disk and any format type gui and terminal both.
    – aeroxy
    May 5, 2017 at 7:34
  • 2
    How does it fail, what is the error message shown?
    – grg
    May 5, 2017 at 8:13
  • @grgarside "formatting has failed"
    – aeroxy
    May 7, 2017 at 13:19

1 Answer 1


The first time you try to format the disk it 'fails' to unmount the volume then unmounts it and returns an error. The second time the drive is already unmounted, so the erase usually succeeds. Since I've seen this on new macs and new installations, I think it is a Disk Utility bug.

An easy workaround is to unmount the volume you want to erase before erasing.

Using the diskutil command in terminal doesn't give the same error.

  • "Using the diskutil command in terminal doesn't give the same error." It does.
    – aeroxy
    Jul 25, 2017 at 2:39

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .