I'd use a simple batch file for the Microsoft OS systems, but the Mac system wont like a batch file. I am not sure of any that run on both platforms so your likely looking for 2 different ways to do this.
As far as automated, you could have both external drives attached to the system and either have a scheduled task do this or add it to the startup folder so that when the system boots it syncs both drives.
Here is one that I have used that is a simple batch file between XP and 7 systems:
With my external drives of same capacity and at letters F: and G: , I perform an xcopy from one to the other, and then the other back to the first in case anything was added to the other and not the first.
xcopy f:\*.* g:\*.* /s/d/y
xcopy g:\*.* f:\*.* /s/d/y
This works for XP and 7 systems, but haven't tested it with 8 or 8.1 yet.
Windows 7 introduced Robocopy and so I would perform the same routine but with Robocopy instead which would work with 7 and 8 or 7 and 8.1. If windows 8 or 8.1 has legacy support for xcopy then the method above will work, otherwise it will error out and you will have to make a batch file with robocopy vs xcopy, and robocopy has different switches than the /s/d/y of xcopy.
The amount of time it takes to sync up all depends on how much data needs to copy from one to the other. If the drives are almost completely a match with exeception to some added data, then it would be completed pretty quickly, however if your syncing up one drive to the other and the other is freshly formatted and you have like 400GB of data, you will pretty much start this and check on it an hour or so later to see if it is done if USB 2.0. If its USB 3.0 then it would complete sooner, and I am guessing your not running USB 1.0 or 1.1 because of the OS you specified, but USB 1.0 or 1.1 would take forever and a day compared to 2.0.
Lastly some systems and external drives have an eSATA option in which you have a lightning fast connection between the system and the drive(s) and so with eSATA it would be completed very fast depending on the drives attached or SATA controller being the bottleneck of speed, but this would be way faster than USB 2.0
