"I don't know the best way to do it on a mac?"
Easy, and cheap, too.
a. Get one of these:
http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias=aps&field-keywords=usb+sata+dock&x=0&y=0
(many items shown, they all work the same, pick one you like that's cheap)
b. Download CarbonCopyCloner here:
http://www.bombich.com
It's free and one of the best pieces of Mac software out there.
c. Get a "bare hard drive" from the vendor of your choice (I like newegg.com).
Once you have these things, do this:
1. Put the drive in the dock, connect the dock to the Mac, turn it on.
2. The drive will need to be initialized, use "Disk Utility" to do this. I suggest that you use a "GUID" partition map with one or more partitions (your choice).
3. After initializing, the drive will appear on the desktop. Launch CCC. On the left, select your source drive (will be the internal drive). On the right, select your target drive (will be the docked drive).
4. Choose a full backup for this first time. After the full backup is done, you can periodically update it and CCC will do an incremental backup (backing up only what has changed, much faster).
5. When the backup clone is done, TEST it by doing this:
a. Restart the Mac
b. As soon as you hear the startup sound, hold down the option key and KEEP HOLDING IT DOWN. In a few moments, the "startup manager" will appear.
c. Select your docked drive with the pointer, and hit the enter/return key
d. The Mac should now boot via USB from the docked drive (it will take longer, because the connection is slower).
e. When you get to the Finder, you should now be booted from the docked drive. CAREFUL! It should look exactly as does your internal drive! You may need to go to "About this Mac" (under the Apple menu) to verify which drive the Mac is booted from. I suggest you change the desktop picture on your backup drive to something which will immediately identify it as such, less confusion that way.
Once you've verified that you have created a bootable clone, you can restart and put the backup drive aside. I'd suggest at least weekly incremental updates. If you get one of the USB/SATA docks (did I mention that they were _cheap_?), you will find that they come in handy for all sorts of uses....