The rapid development cycle of Ubuntu would be too fast for Tails.
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Ubuntu adds features in ways that we find dangerous for privacy. For anexampleUbuntu One ([partly discontinued](http://blog.canonical.com/2014/04/02/shutting-down-ubuntu-one-file-services/))and the [, see [EFF: Privacy in Ubuntu 12.10,Amazon aAds and dData lLeaks](https://www.eff.org/deeplinks/2012/10/privacy-ubuntu-1210-amazon-ads-and-data-leaks).
Ubuntu adds features in ways that we find dangerous for privacy. For an example, see [EFF: Privacy in Ubuntu 12.10, Amazon Ads and Data Leaks](https://www.eff.org/deeplinks/2012/10/privacy-ubuntu-1210-amazon-ads-and-data-leaks).
Ubuntu is led by [Canonical](https://en.wikipedia.org/wiki/Canonical_\(company\)), a for-profit corporation that takes most of the important decisions.
If it were possible to use the same USB stick with another operating system, for example to store files to use on Windows, a virus in the other operating system could corrupt your Tails.