Speaking with my sysadmin hat on, you'll need to create the user joggler first on the core, using the password that you used to create the joggler user on the Joggler device. A group jogger will be created automatically, based on the standard behavior for useradd. Then chown will work properly. You may need to play with making the UID's and GID's match on the joggler and the core, if there are going to be shared files over NFS; otherwise you may run into permission issues. This is based on a quick read of the wiki page; I don't own a joggler.