Best way to mirror these?

here is my setup. Two macally external Esata drives, both with 1.5TB hard drives in them.

Id like to constantly use one of them, while turning the second on to only backup the first one.

I see in disk util that there is a mirror feature, which I have explored. It seems to have to rebuild the entire drive each time the backup is turned on, and slow too...25MB/s max.

Now, I know the drives do transfers of 100MB/s plus to eachother. Do I have any other automatic options here? I could purchase super duper, but that would just clone the primary to secondary.

?

ive been reading up on CarbonCopyCloner, and it looks like itll function the way i need something to.

you could set up a Raid 1 setup witch will result in an exact mirror all the time. You can see how to do it here The new drive should build once and after that be totally transparent.