Yes, I think you suggested that earlier, too.
Mount the new partition elsewhere (not on /data), so you can copy the contents of /data to it, ONCE.

Then umount it, and try the suggested script command above from the "adb shell" command line, just to make sure it actually works before bricking your handset!

Not sure how familiar you are with the "adb" thing, but I find it pretty useful to get a shell (command line window) into the phone. I generally just do "adb shell" from my PC to connect to the phone over USB, and then I use "su" to become the superuser within that shell. Then do whatever.

The first time you try "adb shell" from a given PC, the phone itself will pop-up a security prompt, which has to be responded to on the touchscreen before it will allow the PC to connect.

Cheers