I just had my hard drive in my Powerbook die. All the applications started to slow down and I heard some really nasty crunching as the disk was trying to pull off data. I immediately hooked up my Powerbook to my iMac via Firewire and rebooted my Powerbook. IAs it was coming back up I held down the 't' to bring the Powerbook into Firewire disk mode. Once up it was seen as an ordinary disk to the iMac. I copied all my data over to my iMac. I got it there just under the wire as the Powerbook was really starting to give some nasty disk errors. All of this was possible because of S.M.A.R.T (Self-Monitoring, Analysis and Reporting Technology) data. From the Apple web site,

SMART Hard Drives Monitoring tools continuously sample SMART (Self-Monitoring, Analysis and Reporting Technology) data from each hard drive to monitor performance and drive health. If the tools locate or predict a problem, you’re alerted immediately.

It saved me and I didn't loose a byte. A 80GB hard drive is in the mail. Cool.