Get this bridge.
It will give you bit by bit mirror.
Here is how...
Change the dial to SAFE
Connect Primary HDD to it
Press the change mode button
Connect the Secondary HDD to it
Within 45sec, data should start duplicate.
Its transfer rate about 75MB~100MB/sec
You should have a mirror within NO time...
If you want to clone bigger or smaller drives, there is way to do it too.
BTW, this is the ONLY HW raid the mirror drives w/o wipe the data that I know.
It's great for cloning drive, including OS drive