What is the life expectancy of a Powerbook G4 hard drive?

I am afraid my Powerbook G4 is beginning to break down. The battery gave out and I had to replace already. Now, I am often unable to boot from the hard drive and have to boot from an external fire wire drive. My Powerbook’s hard drive just plain sounds like it’s dying. When I turn it off, it slowly whirrs to a stop these days. I’d love to get another year or two out of my machine if possible. Does anyone have any advice for me? Thanks.