Backing Up

I have data spread over various hard drives and occasionally one fails.

I think what I would like is two USB external drives in some kind of soft raid configuration such that if I write to one then the data ends up on the other.  I would like the copying process to not slow down the computer so it needs to only happen when the computer is not busy. 

Most of the time neither hard drive would be plugged in so maybe this is not so feasible.  Maybe I just want to plug the two drives in and have one mirrored to the other.

Hmm

OK so I have two drives.  One has data and the other is blank.
I will try SyncbackSE Turns out my licence which was purchased in 2006 is not valid and I am offerred an upgrade.  Lets try evaluation first.