Carbonite user Bruce Greenan was disappointed to discover that the online service's Mirror Image backup option saves to an external drive rather than the cloud. He imagines a scenario where "my external image drive is in the burning house with my laptop," and he loses both.

I love cloud-based data backup. It's easy, automatic, and it stores your data far from your home or office. A single fire or flood can't destroy both the PC and the backup.

But to my mind, backing up Windows itself to the cloud doesn't make sense. The advantages of online backup disappear when you have to restore Windows as well as your library data.

To protect your data, all a backup program needs to do is decide which files to copy, and then copy them. But the only way to reliably back up Windows, the Registry, and your installed programs is to copy the entire drive--or at least the C: partition--as a clone or an image.

Keep in mind that system backup is nowhere near as important as data backup. You can always reinstall Windows and your applications. You can't reinstall your documents, spreadsheets, and family photos. That's why I recommend doing an image backup two or three times a year, while backing up data on a  daily basis.

Let's consider the types of disaster that will make you glad that you have a backup:

First, there are the accidents that result in a loss of certain files. Maybe you emptied the recycle bin before realizing you deleted the wrong folder. Or you overwrote an important project. For these problems, all you need is your data backup--local or online.

Second, there are situations where you lose everything on your drive. Maybe it crashes, or  Windows becomes so messed up that only a reinstall can fix it. An image backup will save you the trouble of reinstalling Windows and your applications. And once the image is restored, you can use your data backup to recover your library files.

In the third scenario, a fire, flood, or burglar takes away both your computer and the external drive. This is where an online backup proves superior to a local one. But in this situation, you'll have no use for an image backup. You'll have to buy a new PC, and  an image made on one computer won't work well on another. Your new computer will already have Windows (or another OS) installed, and you'll have to configure it and install applications anyway.

In other words, as long as you have a safe, online backup of your data, losing both the PC and the system backup is no worse than losing the PC.