Also a big part of this article is copied from BackupTheEFS, it would be better to find a way to include content instead.
See DataPartition for more details.
First, you need to make sure that the data partition is not mounted.
To do that, you can run this command:
adb shell "umount -l /data"
If the /data partition was mounted, it will unmount it. This will look like that:
$ adb shell "umount -l /data" $
If it was not mounted, it will instead show an error that we can ignore:
$ adb shell "umount -l /data" umount: /data: Invalid argument
Once this is taken care of, we can backup the partition with the following command:
adb pull /dev/block/platform/dw_mmc/by-name/USERDATA ./