There is no minimum, Just make sure its formatted correctly with this and choose FULL format with ERASE ON.
Aha, thanks for the clarification! First time I tried Ubuntu on my HD2 couple of months back, it didn't work in my class 2 microSD. Read somewhere it needs at least class 4 to work. Then again, it might be really outdated info since I haven't been following the Ubuntu build for quite a while.
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