I have free sata connector in my laptop and I've wondered if is there any option to use SATA as some kind of data transmission, like USB. Will i need to write my own drivers? Bottom line is: i want to design a device that is connected to laptop in optical bay. Any ideas?
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.