|Make folders on the external. That is what you should do anyway. Don't save file directly to the root directory. If you make a folder called image6309Cdrive then the next time you image name it in the same manner using the current date. Try to keep two images if you can. Make sure when setting up the image that you get the path correct to the folder you made.|
You can also image to DVDR or CDR. Ghost and Acronis will span multiple disks. I use Ghost 2003 and the first disk of an image is boot able so you can restore directly from there.
With an external hard drive you would need to boot to the Ghost CD and then access the external that way. More chance of failure the more complicated things get.
You understand that most files will be compressed when imaging so they take less room. If you have your internal drive partitioned into multiple partitions as I always recommend you can image and restore each partition separately and the others will still be OK and will work.
All that said, imaging to the external is still better than not imaging at all.