use old mac mini as backup
How do I convert my old macmini to a backup storage device
Mac mini (Early 2009), Mac OS X (10.6.8)
How do I convert my old macmini to a backup storage device
Mac mini (Early 2009), Mac OS X (10.6.8)
Connect it via FireWire and boot the old Mini in Target Disk mode.
How to use and troubleshoot FireWire target disk mode - Apple Support
use old mac mini as backup