Will the command "hadoop namenode -format" change the whole hard disk filesystem type?
HDFS isn't a real file system that runs on your hard drive like ext3 or something similar, it is close to database. Just like a database, HDFS stores data on a regular file system and provides an API to access its data.
So don't worry about corrupting your data outside Hadoop with "hadoop namenode -format", this command just initialize the "database".
You can read about HDFS in a book "Hadoop: The Definitive Guide", also you may make the acquaintance with HDFS with this article - http://www.aosabook.org/en/hdfs.html