A los que no han podido instalar CM12 porque les aparece el error: Unable to mount system, Eintenten esto a ver. El desarrollador de la rom dice que esto podria funcionar:
Reboot phone to the recovery with holding vol- and vol+ & power. Plug the phone to the pc. Open minimal adb. type these.
adb devices (you should see your device connected)
adb root (this command is NOT nessesary, but maybe)
adb shell /sbin/busybox mkfs.ext4 -L system -m 0 /dev/block/platform/msm_sdcc.1/by-name/system (this command will format your system. It is not the same what recoverys wipe does)
There… No more commands. Then in recovery, go to file manager->system (it should be empty). Then go back to TWRP main menu and install the cm12 rom again. With OTG or if the file is in your internal storage. The path to the internal storage is /data/media/0/