[offtopic]
USB Flash sticks have a limited number of READ/WRITE cycles and thus would not be suitable for running an OS off it.
Unless you want a temporary computer
This does not, however, include external hard drives, which probably all use USB, but since they have moving parts instead of the IC surface mount on USB flash drives/pen drives/whatever, they would (somehow) be more durable if an OS was run off it.
[/offtopic]
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