Recently I had to rescue a Windows system and required a new bootable USB stick.
Mounting the Windows 8.1 installation ISO via the GUI brought up an empty folder or a blank README.txt.
I found the solution via superuser.com – the mount helper isn’t detecting the UDF partition on the disk, so you’ll have to do something via the command line like:
mount -o loop -t udf /media/files/en_windows_8.1_with_update_x64_dvd_4065090.iso /tmp/iso
Say your USB stick is on /media/usb and already formatted with FAT32 (vfat), then you can do
rsync -av ---progress /tmp/iso/. /media/usb/.
and you’ll get a UEFI-bootable Windows 8 installer (that can also be used to get into a recovery environment).