SATA as transport medium for external device
There are low cost readily available IDE/PATA to SATA convertors such as this. 


The PATA interface is straightforward and very well documented.

All the versions I've seen will support the PIO and UDMA modes, and clock speeds are quite reasonable if you stay at or below UDMA(2) -- 33 MB/s. 
You can make a very simple interface using the PIO modes, this will give you only a maximum of 16.7 MB/s but it's very easy to emulate a disk drive in these modes directly from an MCU.

This might be a good starting point for you.