But pulling up the manual for the motherboard told me that there was a M.2 PCIe slot built in. And, as it turns out, although I have mirrored 3 TB hard drives in the box, I have only used about 300 GB of storage on the whole machine. So everything that I have will fit easily onto a terabyte M.2 drive.
And I have a backup program running to keep copying everything out to my NAS.
So I ordered a terabyte M.2 drive. It arrived today. I unplugged the computer, swept up the assorted dust rhinos on my desk, and went to install the new drive. Happily, I was able to pull the motherboard box off the shelf and find the M.2 screw and standoff that I'd tossed back in there when originally building the machine.
I'd never plugged an M.2 drive in before, but I finally figured out the right angle and got it in with no problem.
WD provides a free download of software to clone your existing drive to the M.2 and -- after a few hours -- the operation was done.
Things seem to be loading pretty darn fast, which I find encouraging. :)