Failure in file transfer with sftp

[DarkSkull]

I Raspberry Pi 2 as a Torrent Box. I recently attach to it an external usb HDD formatted in NTFS. I can download successfully any file in the hardisk but when I try to connect to my Raspberry with Filezilla and trying to download file stored in the external disk I got this error message:

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 error while reading: received failure with description 'Failure'
 Transfer Failed

I also run the command

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df -h

to see how much free space I have and this is the output:

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 File system     Dim. Usati Dispon. Uso% Montato su
    /dev/root       5,9G  3,9G    1,8G  69% /
    devtmpfs        458M     0    458M   0% /dev
    tmpfs           462M     0    462M   0% /dev/shm
    tmpfs           462M  6,5M    456M   2% /run
    tmpfs           5,0M  4,0K    5,0M   1% /run/lock
    tmpfs           462M     0    462M   0% /sys/fs/cgroup
    /dev/mmcblk0p6   65M   21M     45M  33% /boot  
    /dev/sda1       233G  875M    233G   1% /media/pi/external-hardisk
    tmpfs            93M     0     93M   0% /run/user/1000

I don't have any problem with the space.
The permissions of the folder are given recursively and are:

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drwxrwxrwx  1 root root 4096 dic 16 17:30 external-hardisk

So, what is the problem?