So this external disk sits where? In your house? Do you move around with your backup where you might loose it?
If you don't move about with the disk, you have physical security of the device - who in the house that also has physical access to the device are you worried about seeing the files?
I bring up these topic because in all honesty its rare that files actually warrant encryption - yeah its cool! Yeah you should secure your files, but the issue is more often than not encryption secures them from the owner of the files more times than securing them from unauthorized access
If you have some sensitive work files - sure by all means put them in an encrypted volume. But do your other files warrant encryption, keep in mind that if something goes wrong, you forget your password, loose the encryption key file, etc.. You have no loss access to these files as well.
You mention they are backups - in the same house I assume, so the originals are encrypted? Kind of pointless to have files encrypted in the back, next to the originals - if the originals are not encrypted.
What encryption do you use for the originals - why not use the same for your backup?
Look around even just the threads on neowin, you get people jumping on the encryption band wagon because well it's cool
Next thing you know - how do I access my files, I used EFS and then reinstalled my OS - but didn't bother to read all the details and never backed up my encryption keys. Now I am F_cked!!!
Here is a basic question before use of encryption -- would you rather loose the files, then someone access them?