Hard drives
This article discusses the monitoring of hard drives and the best way to deal with hard drive failures. It explains how to make a complete backup of your Gentoo system into another hard drive.
Monitoring your hard drives using SMART
Modern hard drives come with a self monitoring system called SMART. This system will apparently report problems and can detect in advance about 2/3 of hard drive failures. Under Linux, a set of tools, smartmontools, may be used to diagnose your hard drives. You can use the program smartctl manually, or install a daemon that will always run on your system and report failures.
Backing up your data and system: making a stage4
Under Linux everything is a file so it is easy to create a complete backup of your system. With Gentoo the best way is to generate a personal "stage4" file. Follow this guide on the Gentoo Wiki to do just that.