Systems Deployment

I am trying to store my harddrive image to external hard drive through virtual floppy technology. I am able to boot my computer to dos mode and to the imagecenter through executable made from the virtual floppy (.vdf) file, but only when I don't attache external hard drive to the computer. If I attach external drive (I am using SCSI drive) to the computer, and double click on same executable which I used when external drive was not attached, my computer doesn't boot to dos and doesn't load imagecenter. I am using deploycenter's boot disk builder to make virtual floppy and the executable as well. Is this problem related with external hard drive or is related with the virtual floppy I have created?
ANY HELP?