[Replicant] I9300 backup of /efs not yielding usable image

[Tad]

That is a very weird way to backup a partition.

Please instead try the following:

$ adb shell
$ su
$ dd if=/dev/block/platform/dw_mmc/by-name/EFS of=/sdcard/efs.img
$ exit
$ adb pull /sdcard/efs.img

To restore use Heimdall or Fastboot.
You can maybe do it running by:
$ adb push efs.img /sdcard/
$ adb shell
$ su
$ dd if=/sdcard/efs.img of=/dev/block/platform/dw_mmc/by-name/EFS
$ reboot

You can also use adb root instead of su if not available.