I have a board with one mPCIex fullsize and the other one halfsize. I was wondering whether do I need the fullsize slot for mSATA drive or the halfsize slot would suffice ?
The exact drive would be: Adata SSD PremierPro SP300
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According to the Premier Pro SP300 page:
This is the size of a full-sized mPCIe card.
But, it's not just slot size you have to worry about: mSATA is not equal to mPCIe, even though the slots are the same (or at least similar).
So whether it will work at all will depend on your motherboard.