Hello @rctneil, welcome to the forum!
I’m also running Dupliacti on unRAID (official Docker container) but am not backing up to USB (just an array disk and cloud storage).
Depending on what version of Duplicati you were using, you may have been bumping into a bug where the progress bar wasn’t being updated correctly even though the backup had progressed past the verification.
Note that while warwickmm is correct about the end-of-backup “can I download and verify a file” tests, I believe the issue you’re facing is the pre-backup “does the destination have all the files I think it should have” check.
If updating to a newer version of Duplicati doesn’t resolve the issue, consider TESTIGN with this parameter - if it resolves it, then we’ve isolated the issue and can move forward from there:
--no-backend-verification
If this flag is set, the local database is not compared to the remote filelist on startup. The intended usage for this option is to work correctly in cases where the filelisting is broken or unavailable.
Default value: “false”