First, there's moving hte library which means all files and folders, and there's moving just media which leaves the library files and others on the main drive.
iTunes: How to move [or copy] your music to a new computer [or another drive] - http://support.apple.com/kb/HT4527
do not confuse moving your whole folder and library with moving just media files as in
iTunes for Mac: Moving your iTunes Media folder - http://support.apple.com/kb/HT1449
Quick answer if you let iTunes manage your music: Copy the entire iTunes folder (and in doing so all its subfolders and files) intact to the other drive. Start iTunes with the option key held down and guide it to the new location of the library.
You say you edited the library location which suggests to me you did not move hte library but just told itunes where to put media. It would help to know specifically what you did.
If iTunes cannot find the library file in the location you indicate on an external, it defaults back to the internal. Make sure the external is on and mounted before starting iTunes. I see a number of issues where people mention NAS and iTunes and it just seems to be one of those things that's a bit buggy with iTunes at times. Maybe the connection isn't fully established, or something such as sleep is dropping it.