Most reliable way to mirror large number/size files/folders between drives & computers

rsync has proven to be very stable and effective throughout its years of Unix service. It’s included in any OS X since Tiger and it’s relatively easy to use.

If you want the bonus of a GUI, you can use Arrsync, a free utility better described as:

A Mac OS X graphical front end for the utility rsync.

If, on the other hand you want something more “easy to use” I can recommend you take a look at the two winners:

SuperDuper! ($)

Carbon Copy Cloner (Free, Donationware).

Either will allow you to clone entire drives or folders or anything you can imagine.

Their reliability to copy permissions, ACLs, and that stuff has been proven throughout the years.

As an alternative to CCC or rsync, you could check out ChronoSync; I've been using it for years and it's just rock-solid.