I host a website and 4 domains are mapped to the domain that hosts the files. Theres lots of sites linking to one of the mapped domains, but not to the domain that actually hosts the site files. Is this OK or should I make the domain that receives lots of links the primary domain? Are there any SEO tips for domain mapping (white hat)?
Linking filenames is not very useful, you could try to give your subdirectory which it have incoming link on your own site or you could linking to your categories.