Does the late 2013 Retina Macbook Pro Support PCI-E SSDs? I'd like to upgrade my late 2013 retina Macbook Pro with a M.2 PCI-E SSD. Is this possible? Do I need an adapter?
As you can see in the image below from ifixit, the 2013 Retina Macbook Pro does have a kind of M.2 PCI-E SSD. (It is the black rectangle the person is holding.)



However Apple uses proprietary firmware on those SSDs, so only SPECIFICALLY Mac compatible drives will work such as these these and these.

This Ifixit guide shows you how to install a new drive, and links to a page that lets you purchase a compatible drive.