Well, it might work.
And son of a ---smurfette---, it actually worked. I took the old hard drive out of the Xbox case so that I could get the power and IDE cables as flat as possible, reduce any possibility of connector jitter and tugging when I did the hot swap. Let it run overnight, and now I've got an old Xbox with a 7200RPM 2TB SATA drive in it. Seems like UnleashX doesn't recognize that 1.7TB G: partition I have now, but it's working fine without any additional configuration in XBMC. I did have it initially running as an exact clone of the old 160GB drive, but then I took a shower and stood under the water muttering to myself, "what would they think of me if they knew I left 1.7TB of space on the table...."
And, now, mostly for my own future reference when I search on here to remember how I did it:
If you’re running a fairly recent bios, the dvd drive check is probably disabled and you can boot up with both hard drives attached and no need to hot swap.
When you get into Chimp, first “scan physical ide devices” and make sure both drives appear.
Then do “master drive options” and pick partition information. If the partitions don’t show up, run the XBPartitioner tool and choose the option that matches your setup. Probably “F takes remaining space” if your old drive isn’t too large. If you don’t do this, your F partition won’t copy over in chimp.
Use the option in Chimp 261812 - Clone Master to Slave -> Selective (Select Partitions) -> C, E, F, & G takes remaining space on a large new drive.
Only performance difference I noted was a few more seconds of initial boot time when the system comes on, but I think that's the adapter hand shaking between the Xbox and HD. Once booted didn’t notice any difference.
This was the adapter I used, HDE RXD-62987-7
https://www.amazon.com/gp/product/B00JVUXMRI/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1Put some tape on the back of it unless you like seeing sparks fly.
EDIT - the new drive needs the SATA adapter jumper set to "Slave" when you're copying the old drive. Change it to "Master" when you install it into the Xbox.
PS: In a second Xbox, when I upgraded a 30GB drive to 320GB, I couldn't figure out how to make the F partition on the new drive large - it always made an exact clone of the F partition and left ~290GB unusable. So I used the options above and ended up with an exact clone of C, E, and F and then a new large partition G. In my case, it wasn't a big deal because that F partition was ~20GB and has almost everything I want on it already, but the rub is there's only like 100MB free on it. XBMC works fine with a G partition, but you'll need to update the paths on your Apps/Video links to point towards it. I've noticed that XBMC doesn't like two copies of the same game on two different partitions. It'll work if you use the file explorer to run the default.xbe, but if you try to navigate to it through the Apps shortcut it's not working. It shows the game one time and doesn't have the icon. You can probably get around this limitation if you use different folder names or something.