Adding a twin drive after the first has data

[andycr]

I have a 2GHZ Core 2 Duo laptop with 2GB of RAM converted to a FreeNAS 8 box, and have a quick question. It's doing a great job of serving files off the 3TB hard drive attached to it, but I'd like to add a second, identical 3TB drive which is currently making its way to me via UPS. I'd like the second drive to simply be an exact copy of the first, for redundancy.

As you can imagine, the first drive already has data on it. So I'd like to get the new drive, plug it in, have it copy the contents of the first drive over, and simply keep them in sync. What's the best way to accomplish this?

Thanks!