Tails Installer fails to mount certain USB sticks in Tails

I’ve seen 2 people on #tails report a similar issue. They create a USB key on Windows, then boot into Tails and try to create the final USB key. But it fails, due to a permission issue.
Executing tails-installer-launcher as root solves the issue.

Somebody on the channel suggested that this might be due to sticks which are formatted in NTFS, but I don’t know if that is correct.

Here is the output from one of these people:

DEBUG=1 tails-installer-launcher
[creator.py:1318 (get_extlinux_version)] WARNING: extlinux not found! Only FAT filesystems will be supported
Traceback (most recent call last):
  File "/usr/lib/tails_installer/tails-installer", line 105, in <module>
    main()
  File "/usr/lib/tails_installer/tails-installer", line 99, in main
    win = TailsInstallerWindow(opts=opts, args=sys.argv)
  File "/usr/lib/python2.7/dist-packages/tails_installer/gui.py", line 325, in __init__
    path=config['running_liveos_mountpoint'])
  File "/usr/lib/python2.7/dist-packages/tails_installer/source.py", line 67, in __init__
    self.size = _dir_size(self.path)
  File "/usr/lib/python2.7/dist-packages/tails_installer/utils.py", line 57, in _dir_size
    for item in os.listdir(source):
OSError: [Errno 13] Permission denied: '/lib/live/mount/medium'

Related issues

• Is duplicate of #13206 (closed)

Original created by @u on 11280 (Redmine)