I thought I'd post back and let everyone know what a doofus I am.
I decided to take the 2tb green drive out of my DVR expander since it is a 3gb/s drive that can be jumpered down to 1.5gb/s. I put it in, and it still didn't show up. It was then that I realized I wasn't seeing any SATA ports in the bios. I poked around and realized I couldn't find ANYthing related to SATA in the bios anywhere. I pulled up the motherboard manual and dug around, and it turns out the SATA controller wasn't enabled. Doh! The spot to enable it was buried down in a menu path I hadn't looked at while poking around.
I enabled it and put the new 6bgp/s drive back in. It was recognized right away and it's now formatted and ready to load up with all kinds of good stuff.
Even though my SATA chipset doesn't support auto speed negotiation the drive was able to manage. I had been looking around online and found all kinds of reviews for this drive saying folks had gotten it to work with 1.5, 3.0 and 6.0 gb/s controllers without doing anything, so it was bugging me that I couldn't see it at all anywhere in the bios when it was installed.
So looks like my cheapo $149 self-built NAS from 2007 will continue to live on (now I'm off to search for a gigabit NIC).