This is usually due to an unresponsive peripheral. Try powering everything off. Then, simplify things as much as you can, power up and try again.

It worked!
 

Hey David,

Just wanted to let you know that powering down my whole system and then powering it up again seems to have done the trick. The backup script is now happily buzzing away. Thanks again for the technical support. I'm a big fan of your software, by the way...

Glad I could help, and that you're a fan! :)