DOH! I was wrongly remembering it being the CENTER button, it's the LEFT as BigBambi points out. Please disregard this post.
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Been using RB for months and no problems till today. Just updated to r16353-0802 and notice that I can't boot into the original OS now.
I used to hold the large center button while turning the unit on and boot into the original Sansa OS, it won't work now. Tried pressing just after starting it and holding it down while starting it - neither will work.