Unexpected error message for missing target folder

Thanks for testing. After trying to get documentation (this is Windows Vista era, and seems to be
fading away), I’m thinking that its available controls might not bear much similarity to OneDrive’s.

The Windows Drive Letter Problem has a possible solution to detect not-really-there destinations.

  --alternate-destination-marker (String): Look for a file in the destination
    This option only works when the --alternate-target-paths option is also
    specified. If there are alternate paths specified, this option indicates
    the name of a marker file that must be present in the folder. This can be
    used to handle situations where an external drive changes drive letter or
    mount point. By ensuring that a certain file exists, it is possible to
    prevent writing data to an unwanted external drive. The contents of the
    file are never examined, only file existence.

If that’s not sufficient, but there’s still some way to tell, you can write your own run-script-before.