when you add an external drive to your mac laptop as a backup device, go with two partitions, one for time machine backup, and one for a bootable copy of mac os x. put a copy of the mac os x installer in the bootable mac os x partition. when you do need to restore you can boot from the external drive, reinstall on your new or newly wiped internal drive and restore from the time machine backup.
if you go with this setup, you also have the option of restoring your backup onto the external drive while you wait for the replacement hard drive to arrive from your vendor of choice. it won't be much of a laptop if you have to lug an external drive around, but at least you'll be on the air.
don't attempt to defrag in place. wipe and reinstall from backup. it was defragging in place that put me into the 36 hour rebuild from hell.
