Hi...
I have cloned drives and currently am running one.....
For a total system back up it is best to create a boot CD after all how are you going to restore your computer if your hard drive fails , you will have "No" operating system and "You can't boot from the external drive"....
If you can spare your external drive for the "only purpose" of keeping a cloned copy of the current drive then make a copy application boot CD disc , EaseUS have other free software for this purpose (I have and use this ).
http://www.easeus.com/disk-copy/
Cloned drives can't be connected to your computer when booted normally. or the drive letter will change from the original to one allocated by your computer , that why if you commit the external drive
as back up for your system image then that is all it could be used for ...
After you have created a total back up image of the current drive with the disc copy boot CD to the external drive NEVER connect this drive to the computer again , "When windows is booted"..
In the event your currently installed hard drive failed and having no operating system.
After purchasing a replacement internal hard drive for your computer you boot from the copy disc CD and have the external connected then clone the image to the replacement drive....
The replacement drive will now be a copy of the original , retaining the C drive allocation and boot able as the original.
Hope i have been of help..
PS Have gone from 80GB to 320GB to 750GB ...all cloned !