Backups work best when you have multiple copies, at least one of which is both current and offsite. OS X's Time Machine feature plus high-capacity, low-cost hard drives make it possible to back up regularly and to rotate drives through backup sets and store a full backup somewhere away from the data that's on it.

But what about when the worst happens? When fire strikes, a lightning bolt fries your computers and backup drives, or a burglar runs away with the goods? A drive stored offsite helps, but the files stored on it are out of date the second it's unplugged and lugged away. An online backup service can be the perfect addition to your backup plan.

Here's a look at eight services with OS X software to manage automated backups.

How we tested

Testing a slew of online backup services isn't for the fainthearted. Using a Comcast Internet connection that's described as 15 Mbps downstream and 2 Mbps upstream, but frequently provides far higher rates, we installed each of the software packages and selected at least 10GB of files to back up, up to 100GB in some cases. We used the services for a few weeks (in the case of Mozy and CrashPlan, I had been using the services for years and months, respectively, and examined large current backup sets as well as performing additional tests) and tested restoring backed up files as well.

How they work

All the hosted backup services we looked at use OS X software to synchronize data on one or more of your computers with their hard drives and services elsewhere on the Internet. All support OS X 10.4 and later (including Snow Leopard, although come companies have noted minor compatibility issues), as well as Windows and, in some cases, Linux.

The services store your data on massive server farms that might have hundreds or thousands of terabytes of storage. Companies provide few details on their Web sites about where and how they store data. Jungle Disk is unique in relying on cloud-based metered storage, with a choice between parent company Rackspace and Amazon's Simple Storage Service (S3).

Jungle Disk

After the initial backup, the services are clever about sending changes. Rather than uploading a 10 MB file again, or even a 10KB one, the software on your computer breaks a file into pieces, and then creates a mathematical summary of each piece. That summary is compared to what's stored on the server, and only changed or new pieces are transferred (JungleDisk requires a Plus subscription for this option).

That process also allows an older version of a file to be reconstructed, using a base file with any intermediate changes patched on top to reach the version you want. These piecemeal updates are also typically compressed to make the upload even faster.

Deleted files may be deleted immediately from the backup set, or retained as part of stored older versions forever or for a certain number of days. Most software lets you choose how this is handled.

Every service has its own limitations regarding how it finds, packages, encrypts, compresses, uploads, and receives changed files, which can reduce bandwidth below your maximum upstream rate. In testing with each services on a cable Internet connection that regularly tops 5 Mbps upstream, each service was able to store several gigabytes overnight.