A short 24 hours before Michelle Warren planned to back up her computer her hard drive died. “The photos were gone. The music was gone.” And some of her work files, too, says the principal of MW Research and Consulting, a technology market research firm.

It’s a devastating blow to anyone with digital data—from photos, to contacts, to budgets, to music. All of it can all be wiped out with a single hard drive crash. One solution is to save a backup of your hard drive in partitions or external drives. But what if someone breaks in and steals your computer? Or what if there’s a fire? Chances are a thief would take the backup drive too, and a fire would destroy everything.

Online backup services
A safer alternative is an online backup service. Pay a monthly or annual fee and this service will automatically back up your hard drive over the Internet to a secure data storage facility, which could be thousands of kilometers away.

To find the right service for you, first decide on how much data storage you need (a quick way is to look at how big your computer’s hard drive is). Ideally the service you choose would have enough space to save the entire hard drive.

Once you know how much space you need, consider the options. Some are free, like Microsoft’s Live SkyDrive, which offers 25 GB of storage space, IDrive with 5 GB, and Mozy’s MozyHome with 2 GB. But free versions usually aren’t big enough for your entire hard drive.

For more space, consider paid options, such as Acronis’s True Image, Carbonite, and the paid versions of IDrive and MozyHome. These services offer more capacity: IDrive at 150 GB to 500 GB, Acronis at 250 GB, Carbonite and Mozy with unlimited storage capacity. Fees are charged per month or per year. For the most part, the more you pay up front, the less you pay overall. For instance, Carbonite’s service costs $4.95 per month if you pay month to month. (All prices in U.S. dollars.) But if you buy a three-year subscription (at $129.95) it works out to $3.61 per month.

In our review, the lowest-priced service was Carbonite at $129.95 for a three-year subscription. But price isn’t the only factor. According to Consumer Reports, it’s also important to consider speed of service and customer support—and, in their assessment, Carbonite performed more slowly and its customer service support is less helpful compared with Mozy.

Double the protection
One advantage of many of these services is that they can back up your hard drive locally, to a second external hard drive, as well as backing it up over the Internet to a remote storage facility. Mozy offers this service, called 2xProtect, at no extra cost. Acronis also offers this option, but you have to pay for the Acronis software ($49.95) and the online subscription ($4.95 per month).

Which one is best?
In terms of speed, MozyHome is faster than Carbonite. It’s also less expensive than Acronis, and offers more space than IDrive, so we believe it’s the most cost-effective backup service out there for PC users.

For Apple users it’s a different story. Mozy’s 2xProtect doesn’t work with Macs (although, like Carbonite and IDrive, Mozy’s online-only backup does work with Macs). For Mac users, then, we find that Carbonite is the most cost-effective choice, since it’s slightly less expensive than MozyHome.