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You can only install an SD/USB image into an SD/USB flash drive. If you install other images to an SD/USB flash drive, then the image will not bootup correctly.
There are several ways to install the OS into the external storage, e.g. TF card, USB storage, etc.
Simply select an image and it will automatically identify your flash drive and burn the image. (Recommended)

* Alternatively, utilise the `dd` command from within the Ubuntu Terminal:
$ sudo dd if=/path/to/image of=/dev/sdX bs=8M