Firstly, your computer was not slow because it had to "load up" all your songs.
When a computer starts it only starts up Windows and a few programs, it does not load up personal files (like music files) from your hard disk, it will only load them up if you start itunes.
Secondly, having your music files on an external hard drive is only slighly safer than having them on your main hard disk.
It IS a good idea to separate your personal files from your Windows files, so in that sense it is a good idea, but your external hard drive still has moving parts so could crash at any time.
And in fact being external it is more likely to get dropped (or maybe stolen) so in that sense it is not that safe.
The only TRUE safe backup (well fairly safe) is to backup files onto CD or DVD.
You can still keep them on your external hard drive, but if that ever crashes or gets stolen you will still have them backed up to CD or DVD.
Sorry, cannot answer your question about the disk being locked.