Why do I have to format external disk twice? (First time will always fail) 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?
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.