Be wary of cloning disks, because if you run 2 identical operating systems on 2 different PC's Microsoft will screw over your Windows Genuine Validation certificate and nullify both your operating systems.

If you intend to use it as a backup, then I would suggest you backup your files and not the drive in its entirity. Create a backup schedule so once a month you have a full backup and once a week to do an incremental. All the tools to do this are FREE and are included in System Tools within Start | Programs | Accessories on most Windows Operating Systems.