After 366 days of uptime, my dear Debian server sent me an email via SMARTd and you can imagine my joy when I read the title ("back up data NOW", haha).
- use hddtemp to monitor the HDDs temperature (i.e. via syslog); if this goes up, your drives will start singing different tunes and none is pleasing to your ears/data :-)
- install SMART monitoring tools
- employ mirroring / redundancy via RAID 1, 5, or better