Do not tile directly to the plywood. The wood will swell and contract with the seasons. You need a membrane to absorb these forces. Noble Deck is my preferred exterior membrane; Ditra next. Keep in mind some membrane manufacturers want a cement base (that rules out Hardi backer) under the membrane.
You also need a premium thinset. Laticrete Platinum or Kerabond+Keralastic liquid.
A porcelain tile rated for exterior use is a great choice.
Always plan on the worst case scenario when tiling outside. Why do it over?
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