On YouTube, there is a good guide, as the housing can be easily opened using a hair dryer (it's using some double-sided adhesive tape glued on the back). The built-in hard disk is inserted only on the connector and can be replaced by a SSD.
I have a Samsung 840 Pro and taken to come in the Buffalo "housing" in an iMac to 350-450 MB / s read / write. This may not be the absolute maximum that can make the SSD (theoretically), but in my MacBook Retina still faster than about the SSD Pro. This I saved relatively cheap SSD reconstruction of my iMacs and now use the external Thunderbolt disk as a system disk.
The hard disk that was used in this case - admittedly not the fastest - can be divided into a USB3 empty boxes (available here at Amazon for 10-12 euros) still good "second use".
But, as I said, with the loss of warranty you will have to live.
