Have installed Deja Dup to backup data before attempting to upgrade from Linux Mint 18.3 to 19.0
I am directing Backup to an external Seagate HDD but it is crashing with the following report:

File "/usr/bin/duplicity", line 1532, in <module>
with_tempdir(main)
File "/usr/bin/duplicity", line 1526, in with_tempdir
fn()
File "/usr/bin/duplicity", line 1380, in main
do_backup(action)
File "/usr/bin/duplicity", line 1508, in do_backup
incremental_backup(sig_chain)
File "/usr/bin/duplicity", line 653, in incremental_backup
bytes_written = dummy_backup(tarblock_iter)
File "/usr/bin/duplicity", line 232, in dummy_backup
while tarblock_iter.next():
File "/usr/lib/python2.7/dist-packages/duplicity/diffdir.py", line 523, in next
result = self.process(self.input_iter.next())
File "/usr/lib/python2.7/dist-packages/duplicity/diffdir.py", line 195, in get_delta_iter
for new_path, sig_path in collated:
File "/usr/lib/python2.7/dist-packages/duplicity/diffdir.py", line 280, in collate2iters
for relem2 in riter2:
File "/usr/lib/python2.7/dist-packages/duplicity/diffdir.py", line 354, in combine_path_iters
refresh_triple_list(triple_list)
File "/usr/lib/python2.7/dist-packages/duplicity/diffdir.py", line 341, in refresh_triple_list
new_triple = get_triple(old_triple[1])
File "/usr/lib/python2.7/dist-packages/duplicity/diffdir.py", line 327, in get_triple
path = path_iter_list[iter_index].next()
File "/usr/lib/python2.7/dist-packages/duplicity/diffdir.py", line 239, in sigtar2path_iter
for tarinfo in tf:
File "/usr/lib/python2.7/tarfile.py", line 2508, in next
tarinfo = self.tarfile.next()
File "/usr/lib/python2.7/tarfile.py", line 2350, in next
raise ReadError("unexpected end of data")
ReadError: unexpected end of data

did you check if there was a disk full or something similar (during execution)?

I don't know which tempdir this python script is using, but as the first error is about that tempdir [with_tempdir(main)] it may be using a tmpfs which runs out of RAM.
The "main" probably is a pointer to the previous set (or created) tmp directory.

Thank you for your response.
I believe the disk was clear but will reformat to be absolutely certain and try again.
I had deliberately selected as "small" file as a test. Will see what happens.