External HD Sync for Mac OSX

but then I re-read you post.

What is your current backup strategy?

Here is what I would do.
Use one of the external drives as the Time Machine drive. Let it do all the hard work of deciding what needs to be backed up and what doesn't.
Now you have a backup that you can recover from once you have reinstalled the OS.

Then I would consider trying to get something like Carbon Copy Cloner to make a Clone of the Time Machine drive and store it on the second HD.
Alternatively, I would have CCC clone my internal HD to the second external on a Daily Basis.

That way I would have the instant availability of Time Machine's restore function and a Bootable clone of my internal for those pesky "there's smoke coming from my HD" moments.

P