A Blu-Ray drive to add to the MacBook 7,1?
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Which internal Blu-ray drives are compatible with a unibody Macbook Pro?

Be aware that the OS itself won't play back BluRay movies; Steve Jobs was on the record as saying that BluRay's DRM were "a bag of hurt". The linked question has among its answers a link to a supposed workaround for this, but that link no longer works.