[Question] back up via MTK tools

[ashknight]

hi all,
i got a question..
why my s820 cn version does not let me do the backup via MTK latest version.
it always show error
- nodl_pmt - ERROR : Zero file, no space left on device!

machine lenovo s820 cn version MT6589
kernel version 3.4.67
android 4.4.2
4GB storage
rooted using root genius@KingUser

this is my storage info
the internal storage 2.56GB Available is 0.92GB
the built in storage 1.03GB Available is 0.97GB

also i try switch using the SD card 8GB
the problem still the same there is not enough storage.i check the backup in the phone and it is saved in the SD card not phone storage...so why still the is no space.

also i try different MTK version and back up untill end@error
MTK 2.5.3 backup around 700mb file
MTK 2.4.6 backup around 1.6GB file

i hope someone here can answer,show,guide me how to solve this problem