I have made a bootable backup of my mac on an external drive using superduper. Is it possible to make this backup secure? Either by encrypting the backup or adding a password to it?
What you can do is make an encrypted disk images on your external hard drive. You can see this image as a extra external drive upon your external drive.
If you make it large enough (like 500GB or more depending on the size of your external drive).
You can give the disk image a password (Choose 128-bit AES (and/or 256-bit AES)) and whenever you want to access the disk image, you'll be asked for the password.
In this way, there is a save way to protect your backups with a strong password.