Copy to network share using local disk cache in Mac OS X Lion
My first guess would be that you got a typo in the target path (/Volumes/netshare/path, especially the netshare part) and are actually duplicating the data onto the source drive.

If not, I don't really see the benefit of using rsyncfor a one-off copy operation with limited risk of interruption -> just use cp instead, OSX takes care of the network part anyway if you copy something to a share mounted under /Volumes.

If you absolutely want to use rsync leave off the -PW options for a starter. -W is used anyway unless you sync directly to another server by specifying the target as remotehost:path/to/dir, -P is only needed if the sync runs the risk of getting interrupted.