First, I am assuming you to be using the stable release for the slug (0.2.1.1). If that is the case, then the playlist declaration in the configuration file is being used as the source.
OTOH, should that assumption be incorrect and you are using one of the nightlies subsequent to or including the 20 March itsy package, then the daemon is running with the SQLite backend and support for the playlist file is deprecated. So, it does not matter where you put the .playlist file; it will never be seen, since it is no longer used as a playlist source.
For the SQLite backend (e.g. the nightlies), it will be necessary to make use of an XML-based playlist (i.e. that generated by iTunes), an .m3u file or to manually insert them into the database. Examples for the latter exist in various tarball release notes and in the following two forum discussions ,. Note that there have been reports of problems when using Windows generated XML playlist files. Though this was apparently fixed with the release of the 13 July package.
As far as the .playlist file being available on the external drive, if you have properly uNSLUng the device, then mt-daapd is already installed on the external drive. The default installation path is /opt/etc/mt-daapd/mtdaapd.playlist and, on a properly uNSLUng system, the /opt directory is symbolically linked to the “disk 1” drive at /share/hdd/conf/opt.