You can't copy your hard drive in the way you appear to be trying. You require either a backup program or a ghosting program to do this.
If you are feeding to an external drive, ensure that you are able to read from this drive with no operating system in your computer (eg in case of catastrophic breakdown).
Normally, exteranl devices rely on a function of Windows to be able to function. If Windows is not there, you are stuck.
Why not install a second IDE hard drive inside your machine and do it that way? You will still need software though. There is Windows Backup you could try.