Siguiendo este hilo de XDA ([D620][D620R] Partition Table Tool), he podido redimensionar las particiones quedándome así:
Cito del amigo xander33:
"Hi all and thanks for the guide for partitioning via ADB
Here is my step to have about 6 GB for userdata (i set system partition at 768 MB)
Same step as already explain in the previus post
Just change the dimension of the partitoin
Run phone recovery mode
Plug usb cable pc and phone
run cmd.exe -> adb shell
*show partitions
parted /dev/block/mmcblk0 p
*delete partitions 32 - system, 33 - cache, 34 - data
parted /dev/block/mmcblk0 rm 32
parted /dev/block/mmcblk0 rm 33
parted /dev/block/mmcblk0 rm 34
* create new resize partitions (Maximized only for Custom ROM)
parted /dev/block/mmcblk0 unit b mkpart primary ext4 478150656 1283457280 name 32 system
parted /dev/block/mmcblk0 unit b mkpart primary ext4 1283457536 1388315648 name 33 cache
parted /dev/block/mmcblk0 unit b mkpart primary ext4 1388316160 7795000319 name 34 userdata
* format partitions
make_ext4fs /dev/block/mmcblk0p32
make_ext4fs /dev/block/mmcblk0p33
make_ext4fs /dev/block/mmcblk0p34
*reboot in recovery to update /etc/fstab
reboot recovery
So now it's possible install a new ROM or restore your backup (if it's a Custom ROM)"
Tan solo he seguido al pie de la letra lo que pone
¡¡OJO,solamente sirve para las rom de cyanogenmod!!