Thanks for the encouraging response, Eli. I've ordered a new cable and will report back whether or not it fixes the issue in a few days. Thanks!
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Update (1/10/13):
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So I replaced the cheap optibay I got off of ebay with a the pricier data doubler from OWC today. Now the hard drive in the optibay mounts, reads, and writes just fine. Seems like the original optibay was a dud. I've still got a connector cable on the way from ifixit.. I'll either return it or hold onto it just in case.. haven't decided yet.
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Has anyone had issues with the optibay SATA connectors failing over time? If so, I'll probably hold on to the one from ifixit. I already paid for express shipping on the thing... Oh well. Happy that everything is working (for now...) and that it wasn't a logic board issue like others have experienced.