The negative sign is sometimes used interchangeably with the 'gnd' or 'ground' designation. That is the case in this situation, so long as you're positive that the wall adapter is outputting 12 volts DC and not AC. Yes, it would be acceptable to connect the negative (or ground) line to one of the ground wires on the SATA power connector. The hard drive may not like it if it receives 12 volts at a different time than 5v, like what would happen if the wall adapter received power before the Xbox 360, though. Also, if the new drive draws more than 500 mA of current from the 5v line, then the Xbox 360 won't by able to supply it with enough power. For those reasons, I would recommend using a new adapter like the one you posted, but with SATA power instead of a molex connector on the end. You can use 2 or 3 pronged, it really doesn't matter. The 3 pronged one just adds an earth (or ground) line. During normal operation, this line is not used for anything. It's only there for shock protection, in case there's an internal fault in the power adapter.