I'm just guessing, but I suspect the two paired traces leading from the MCU towards the IC that feeds the USB connector, are the Rx and Tx lines. I would unsolder one end of each of the capacitors circled in the crop below and find vias that connect to them, to which I would solder Rx/Tx lines to connect to the SATA data connector. I've circled a couple that look like possible connections but you really need to use an ohm meter to verify and to locate the other two. You could use the USB connection to supply power to the drive or locate the power lines from the USB connector and use those to connect to the SATA power connector. Let us knows how you make out.