node import with file attachment

I have a csv file with a file attachment field. The files have been created in the appropriate directory and are readable by the server.
However, I get:

Input error: sites/mydomain.org/files/resources/myfile.pdf is not allowed for Resource Attachment (not a file in sites/mydomain.org/files/resources)

But it is a file and it is in the named directory.

Any ideas on what the problem might be?

Many thanks,
Paul N.