I use My Backup Pro to backup my Verizon s3. The default setting backs up data to the internal memory. If you want to backup your data to your micro SD card, you have to enter the following string, under settings, Backup location, when you open My Backup Pro:
The external sdcard path is /mnt/extSdCard <--- case sensitive.
The external sdcard path is /mnt/extSdCard <--- case sensitive.