Comment 2 for bug 1247276

A bit more digging suggests this may be similar, or the same as, bug #1202363.

It seems to be something to do with write permissions on the mounted disk. The folder on the external hard drive (usb connection) has the following permissions:

drwxr-xr-x 1 root root 262144 Nov 13 11:01 Backup

Here's the output after running DEJA_DUP_DEBUG=1 deja-dup --backup from a terminal. Hope someone finds it useful.

DUPLICITY: INFO 1
DUPLICITY: . Writing /media/julian/JULIAN_1/Backup/duplicity-inc.20131024T134023Z.to.20131113T110009Z.vol1.difftar.gpg

DUPLICITY: WARNING 1
DUPLICITY: . Attempt 1 failed: GError: Error setting permissions: Function not implemented

DUPLICITY: DEBUG 1
DUPLICITY: . Backtrace of previous error: Traceback (innermost last):
DUPLICITY: . File "/usr/lib/python2.7/dist-packages/duplicity/backend.py", line 316, in iterate
DUPLICITY: . return fn(*args, **kwargs)
DUPLICITY: . File "/usr/lib/python2.7/dist-packages/duplicity/backends/giobackend.py", line 137, in copy_file
DUPLICITY: . target.get_parse_name())
DUPLICITY: . File "/usr/lib/python2.7/dist-packages/duplicity/backends/giobackend.py", line 112, in handle_error
DUPLICITY: . raise e
DUPLICITY: . GError: Error setting permissions: Function not implemented
DUPLICITY: .

[Repeated 3 more times...]

DUPLICITY: INFO 1
DUPLICITY: . Writing /media/julian/JULIAN_1/Backup/duplicity-inc.20131024T134023Z.to.20131113T110009Z.vol1.difftar.gpg

DUPLICITY: ERROR 50 put '/home/julian/.cache/deja-dup/tmp/duplicity-zDZOK1-tempdir/mktemp-5Ygh_z-3' '/media/julian/JULIAN_1/Backup/duplicity-inc.20131024T134023Z.to.20131113T110009Z.vol1.difftar.gpg'
DUPLICITY: . Error setting permissions: Function not implemented

DUPLICITY: DEBUG 1
DUPLICITY: . Removing still remembered temporary file /home/julian/.cache/deja-dup/tmp/duplicity-zDZOK1-tempdir/mkstemp-An1Bcy-1

DUPLICITY: DEBUG 1
DUPLICITY: . Removing still remembered temporary file /home/julian/.cache/deja-dup/tmp/duplicity-zDZOK1-tempdir/mktemp-5Ygh_z-3