A diretiva Redirect
não usa um URL completo como o local "antigo".
Tente:
Redirect 301 /old.htm http://www.site.com/new.htm
Na documentação :
The old URL-path is a case-sensitive (%-decoded) path beginning with a slash. A relative path is not allowed. The new URL should be an absolute URL beginning with a scheme and hostname, but a URL-path beginning with a slash may also be used, in which case the scheme and hostname of the current server will be added.