The Mystery Connector! Every drive seems to have one, but there’s no standard for what it does - it’s manufacturer specific. It’s for their own use, mostly - for factory testing. Sometimes it can be used in advanced data recovery, or holds a hidden pin for connecting to a status light. Could be JTAG, I2C or good old async serial. But basically no use to you.

As for the not working part - give us a dmesg output when you connect it. We might be able to make something of that.