Just tested it out with the latest stable version of qBitTorrent (same version of libtorrent as Deluge, 0.16.18.0), same torrent, same folder on the same HDD, and it went through without a problem. Could it be a Deluge<->libtorrnt interface problem?

To help troubleshoot: my current "error" torrents all have [ ] characters in them, except for one, that has an extremely long path. However I had several with longer paths than most of the errored ones, but they all had 'websafe' character sets. My guess is there is a substitution going on...

I also appear to have the same problem, however the external 4TB drive I am using is formatted as NTFS. Although perhaps the fact that it is 4TB is worthy of note, as I have come across something recently that showed me that Windows handles a 4TB drive differently than smaller drives. In my case as ...