Q:

Disk Utility won't erase my USB flash drive ("Couldn't open device")

I am willing to dual-boot my MacBook Pro (13-inch, Early 2011) macOS Sierra to install Ubuntu. 
I am having some trouble with my USB flash drive when I try to erase it using Disk Utility.  It says:

Erase process has failed.  Click Done to continue.   
Unmounting disk
  Couldn't open device. 
  Operation failed...

A:

Open Terminal, and type 
$ diskutil list
/dev/disk4 (external, physical):
#: TYPE NAME SIZE IDENTIFIER
0: FDisk_partition_scheme *8000.1 GB disk4
1: 0xEF 12.3 MB disk4s2

It cannot display the USB.   To erase the USB completely, run:
$ diskutil eraseDisk free EMPTY /dev/disk4
$ diskutil list
/dev/disk4 (external, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *8.1 GB disk4
1: EFI EFI 12.3 MB disk4s1

