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Actually, someone has built this into the OS: Apple.  Their Time Machine automated backup software has been part of MacOS since 2007.

Pretty foolproof: you plugin an a not-before-seen external disk, it asks if you want to use it as a Time Machine backup disk.  If you say yes the Mac does one huge backup of everything in the background, then after that it backs up all the files you've changed every *hour*.  Runs completely in the background; so long as you don't need an old file (or your entire disk) back, you never think about it.  If you do need to recover from a dead disk, it will definitely save your bacon (as it has for me and many I work with).

Apple's Time Capsule does the same thing via WiFi or wired connection, and for multiple clients.

As it backs up all files that have changed every hour, one can go back and trivially retrieve older versions of files or even find files that have been deleted entirely (at increasingly greater intervals back in time).

Apple had to make some significant changes to the underlying HFS+ MacOS and file system and, I believe, their whole disk Spotlight indexing system to implement Time Machine.  I suspect that may be harder for Microsoft to do, although I don't know Windows well enough to know.

Note that DropBox takes a bit of a similar approach, but they copy back to their servers only the changed portions of any file in the DropBox folder every time the file is written to disk.  That can make it quite fast over any reasonable speed network connection.  And because the DropBoxed files are always on your local disk as well as the DropBox cloud, one doesn't have to deal with network latency or network offline issues on your working files.  

Arthur Gaer
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On Apr 16, 2013, at 2:53 PM, Tom Fine wrote:

> Thanks for all the information so far. I'm disappointed to be learning that it's still far from background/idiot-proof/foolproof. Why the heck doesn't anyone build this into an OS -- have it back up all the time, you just tell it where to back up to? It seems like the simplest thing in the world -- no one wants to lose data and no one wants to babysit a computer or remember to back things up.
> 
> Is there a Windows batch-command sequence that can just make every file-write be done twice to two different hard drives? Or is there a background-running/non-crashing utility that just keeps two drives sync'd at all times? I do not want to go into the IT babysitting business, I want to make this simpler than it is right now.
> 
> -- Tom Fine