[ubuntu] Error when backing up using Backups

My backups keep failing. I'm backing up to a USB hard drive, and I'm getting the following error. Can anyone tell me what might be causing this? I'm only trying to back up my home folder, not any system folders. I'm ruing Ubuntu 14.04 LTS.

Traceback (most recent call last):
File "/usr/bin/duplicity", line 1494, in <module>
with_tempdir(main)
File "/usr/bin/duplicity", line 1488, in with_tempdir
fn()
File "/usr/bin/duplicity", line 1337, in main
do_backup(action)
File "/usr/bin/duplicity", line 1458, in do_backup
full_backup(col_stats)
File "/usr/bin/duplicity", line 542, in full_backup
globals.backend)
File "/usr/bin/duplicity", line 403, in write_multivol
globals.gpg_profile, globals.volsize)
File "/usr/lib/python2.7/dist-packages/duplicity/gpg.py", line 327, in GPGWriteFile
bytes_to_go = data_size - get_current_size()
File "/usr/lib/python2.7/dist-packages/duplicity/gpg.py", line 320, in get_current_size
retu os.stat(filename).st_size
OSError: [Ero 2] No such file or directory: '/tmp/duplicity-haOyjb-tempdir/mktemp-E1j0jL-3181'