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.

Your initial backup requires you to upload every byte of data. Only one service, CrashPlan Central, lets you jumpstart that process by loading a drive it sends you with up to 1 TB of data ($125 for ground or $145 for 2-day shipping, prepaid both ways).

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).

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.)