Ok you are right what is left is to try.
Size >700G
Now to figure out how to do it safely.
because it is 2 disks and saving both is >1.8 tera
So may be i will buy a new 2 tera hd back up and try

MilesAhead   great job thanks

Thank you all.
tomos to clarify.
I have 2 GENERAL USB DISKS.
They hold general information.(NOT main disk).
The information was never put on both.
But since the information is SIMILAR i need to make them equal
so i will have exact duplicate of my information.(as if i saved information on both).

1. What i am concern is for files with HEBREW names.
2. Long file names (and directories).
3. Problems i could not expect.Since my information is critical.
4. This is why i am looking for a mature software,and not a beta ,beginner .....

P.S see my software which dose not do what i need but is the base
http://limelect.com/downloads/disk-directories-compare/