Adding an external hard drive

My current computer is running XP Professional Service Pack 3 (build 2600)
on a 500GB disk (disk 1).
The disk is formatted NTFS and has only the one partition.

I want to be able to clone my current 71GB computer to a external Hard Disk
(disk 2).
I am going to format my disk 2 to NTFS. (I assume the 4k cluster size is the
default setting on the drop down menu on "Format"?)

If I do this using Acronis True Image 10 saved to disk 2 and disk 1 fails,
do I try and reinstall XP with the CD and then copy back the Image from disk
2?
Am I way off track?
Patti