Somehow, I couldn't save the file which generated by 3rd party app into the external SD card.
I search the answer on web but the possible way to solve the problem is root the cellphone.
I am worrying that it couldn't process OTA update after I root the LG-D838
This is only disadvantage of D838 and this will be big help for me and other owners.
I am also expecting the 3rd party roms, such as AOKP or CM can be released soon^^
Can't wait to flash the phone with other roms