Do you want to clone the drive where it will delete all data currently on the external drive and make it look like the internal drive
do you want to make an image of the internal drive and store the image on the external drive so that if the internal drive failed, you could put in a new drive and simply put the image stored on the external drive onto the new internal drive and it would look just like it did when you originally made the image?
Whew, that was a mouthful!
It sounds to me like you want the second option but let me know for sure. If that is what you want, tell us ...
1. How much free space you have on the external drive.
2. How big is the internal drive
3. How many partitions are on the internal drive and do you want to keep them all in the image (like is there a recovery partition for a factory restore)
4. Can you boot from a USB flash drive and do you have one that is at least 1GB