First, how big is your amount?
Second, what i did is categorize them according to release years. I have folders named 1998 to 2008. I used to find the release year of each track. I know that it's time-consuming but it's worth! I could categorize even more by creating subfolders Progressive, Uplifting, Vocal in each folder, but that's useless and you would get lost even more.
Third, i use both external hard disks and dvds, but the second is safer than the first.