First, you need something to actually back your files up to. A NAS will do, or a USB hard drive. (Or an FTP server, though the syntax will be a little differently). Basically, storage. You can even use this for a folder on the same drive as your original source. Either way, your destination drive will need to be mounted (duh).
Find the location of your source. For me, it's /Volumes/MacHD/Recording
Then, the location of your destination. In my case it's /Volumes/Reaper/Facta
Now, the uber one-liner. Save this as a bash script and kick it off when Reaper closes (I'll get back to this on how to do that), or run it as a shutdown/startup script.
rsync --delete -av /Source/Folder/ /Destination/Folder
You'll end up with something that looks like this:
If you're on OSX, make a new file titled reaperbackup.sh, and in it put the following:
#!/bin/bash rsync --delete -a /Volumes/Source/Folder /Volumes/Destination/Folder
I'll update this a bit more once I get the rest of it done locally, so I have fun and interesting screenshots and shit.