Unable to create backup in TWRP

So I just unlocked the bootloader, got root access and installed TWRP like normal without a hitch but when trying to to create a backup in twrp I am given the following - E:Backup file size for '/external_sd/TWRP/BACKUPS/FA544YJ03048/2016-04-02--08-13-02system.emmc.win' is 0 bytes.

No idea why this has happened or means, any help?

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