IDE is 40pin unless it's a laptop based IDE and thats 44pin. Now if you mean that only 39pins are in use on the cable you can still use it because that one pin isn't used these days and chances are the mac doesn't even use it. Just take a ice pick or a very small drill bit and dill a hole where that pin is on the ribbon cable. I've done it quite a few times and always worked for me.
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