Since you're using Windows, why not try
SyncToy? This utility makes it easy to create a backup of a folder & it's subfolders which can subseqently be updated with any changes. First time out just copy your entire My Music folder out to the external drive. Then create a sync pair between the folder on your hard drive and the one on the external drive. Every now and again you can sync the two folders so that the changes you've made can be copied over to the backup. If you lose anything from your main library you just copy it back manually. I'd recommend you preview each sync op to make sure it's not going to do anything drastic.
I use SyncToy to keep my library synced between a PC at home, a PC at work and a 500Gb external drive that I take backwards & forwards. A degree of caution is needed for this however as copying the iTunes library files between machines is generally not recommended. Backing up locally on the other hand is a good move.
tt2