Unable to mount - problems with TWRP during back up

I have a G900F with the stock rom on it which is running poorly and want to install the Cymogened 12.1 rom.

I have successfully managed to root my phone and install TWRP 2.8.7.0

I cannot make a back up of my existing rom!!!!

when i go into back up on TWRP the first thing i get in RED is

E:Unable to mount 'data'

it then proceeds to try and make a back and then fails

I have tried the following things with no luck
- formatting the sd card in the phone and on the laptop through 3 different formats
- trried to make a back up locally but cant as not enough space
- used the sd card formatter to format the sd card
- a 2gb, 16gb and 32gb sd card with no joy
- run root checker to ensure its fully rooted and it is
- run the micro sd card fix that was disabled in kit kat
- tried to use CWM recovery to make the back up and that did the same thing
-
does any of the below info help

- i have managed to factory reset my phone before without any problems
-i used my phone for work and use gmail and therefore the phone is encrypted , will this have a bearing on me not being able to make a back up to teh sd card or locally

please help as this is driving me insane :)

cheers Bart[/LIST]