I’m having an issue with one of my Duplicati backup jobs. One of the source folders is temporarily missing. In the log I see this error:
"2026-03-11 07:39:33 +01 - [Error-Duplicati.Library.Main.Controller-FailedOperation]: The operation Backup has failed\r\nUserInformationException: Backup aborted since the source path D:\\OLD\\ does not exist. Please verify that the source path exists, or remove the source path from the backup configuration, or set the allow-missing-source option."
I’m considering adding the allow-missing-source option so the backup can continue while the folder is unavailable.
My questions are:
If I enable allow-missing-source and later (after a few backups) the folder becomes available again, will Duplicati automatically resume backing up that folder?
If I enable allow-missing-source, will I still be able to restore files from that folder in the future (for example, one year later)?
I’m using Smart Backup Retention, if that makes a difference.
Yes. What will happen with allow-missing-source is that Duplicati will treat the folder as “deleted” when it is missing. When it shows up again, it will contain “new” files.
The primary purpose of requiring the option set is to avoid cases where you think you are making backup of some data (e.g., removable media) only to later discover that it was not accessible and not backed up.
Generally yes, but it is important to understand that Duplicati works in “snapshots” and each backup run will create a “snapshot” or “backup version”. When you delete old versions, you essentially delete a snapshot/version.
If you are unlucky and delete all versions where the ephemeral source is present, then you will no longer be able to restore the files. There are no options in Duplicati to prevent removal of versions based on content, so it relies on how often the ephemeral source is absent.
How does that happen? For example if you backup C: plus occasional USB D:, splitting backups into an always-there C: and run-when-drive-is-in D: may help.
You can have different retention policy on D: to ensure there’s always a backup version (even if a lot of time has passed since last backup) of the external drive.
You can even use a run-script-before to test for D: drive on a scheduled backup.