The unit I use here is the X200 Tablet, with 120GB main storage + for the Hitachi 160GB experiment HDD. The method I use is exactly the mod method on X220/230, which utilizes an additional SATA pinout in the Dock connector which is usually used for Ultrabay drives. I got the jumper by tracing the "SATA1" pinout from the corresponding dock connector.
I jumper the 4 main SATA pins to the docking pinout , and as a result, you can see for yourself, the Hitachi HDD reads normally, as if we plugged the HDD into the ultrabay dock. For those who are interested in experimenting with strange things like this, I will give you the pinout jumper in picture attached below. Dont forget to cut out the protective plastic shield from the motherboard above the Docking pins, and cover it (preferably using Kapton tape) after you finished (DWYOR).
*edit: wrong links
https://imgur.com/snUnMlS
Recently I realize my cheap SATA SSD is only 1/3 size of its shells, so I decide to take it out, put a kapton tape, and put some thick double-sided tape, and put it in the left palmrest (I need to knock 2 plastic standoff), so Is sandwiched above WLAN card and the palmrest. for the supply voltage, I take the 5v from big "F5" fuse near the left USB connector, and for the GND, you can take it from any exposed screw mount , I directly connected the jumper cable to the SATA pinout instead of using SATA female connector (for space saving, those connector is quite bulky). If you using SATA drive, remember to short all 5V power pin on your drive (SATA power pin number 7 trough 9, see first picture link) or it will be unstable.
For the jumper cable, I use rainbow jumper cable usually used in Arduinos, but I recommend using solid ethernet cable (CAT 5E) instead, so it's more easy to route (not as bulky like my solution)
More Picture
https://imgur.com/a/rQxGiqz
In the future, I want to install an mSATA / m.2 SATA SSD because it is more space efficient + more energy efficient (3.3v vs 5v). maybe I need to get 3.3V from unused Turbo memory slot.




