I ended up removing the disk, copying the files to the pool until the disk was empty, then adding the newly emptied disk back into the pool. This had the unfortunate side-effect of requiring me to re-sync all of my snapraid parity data, however. Not a big deal, but I'd like to avoid that in the future.
What's the correct way to replace a disk, for future reference? If one of my drives blows up tomorrow, I should be able to recover my data with snapraid. But then I have to tell Drivepool to pick up that new disk as a drop-in replacement for the one that blew up. I don't know how to do that.