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#21
There's little difference between plugging in a spare hard drive and 'plugging in' a system image. The reason I prefer a system image is that all the installed software and configuration is guaranteed to be exactly as it was before disaster struck. How much time do you spend keeping the spare HDD up to date and matching your current system? Or how long would it take to reconfigure it once you have plugged it in? Assuming, that is, that you have kept meticulous records of all the changes you made.
About 30 minutes after a disaster I can be back to exactly where I was.