[ubuntu] how to hide ubuntu partition on external HDD?

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http://ubuntuforums.org – Hi! My laptop not enough space, so i have to split 20GB on my external-hdd and install Ubuntu on my external-hdd but when i plug it in another computer they can access 20GB partition and may be i'll lost my data. How can i hide 20GB partition of Ubuntu when using my external-hdd on another computer? :( (Hardware)