John asked if Dropbox effectively works as a backup program, and if using Dropbox and Carbonite together gives him two backups. My answer includes other, similar cloud-based services.
These online services exist for very different purposes. Sync and storage services, such as Dropbox and Google Drive, make selected files available to other PCs and devices, and sync those files between the devices. When you open a document on your PC, it will automatically have the changes you made on your iPad. Cloud-based backup services, such as Mozy and Carbonite, protect all of your data files from possible disaster.
There's a lot of overlap in what they actually do. Both types of services upload your files to another location, effectively protecting them. Both make files accessible anywhere, on any device, as long as you have an Internet connection and your account password.
But neither of them can quite do the other's job perfectly.
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One big problem is the location(s) of what they upload. Sync services require you to put all of the files you want to sync into a single folder set up for that purpose. (You can put subfolders inside it, of course.) To make Dropbox a full-fledged backup, you'd have to move all of your library folders, and possibly parts of AppData, into your Dropbox folder. The same goes for Google Drive and other such services.
But backup services allow you to back up selected folders from all over your drive--or multiple drives. By default, they back up all of the library folders, the desktop, and whatever else they think needs protection. Right-clicking a folder will bring you an option to add or remove it from the backup.
But those backup services don't really sync. One account generally backs up one computer--and if it backs up more than one, it backs up each one separately. You can download files anywhere, but if you alter them, the alterations will not go back into the cloud.
Both Mozy and Carbonite offer sync tools to their paid subscribers. But these require additional software and run completely separate from the backup.
And then there's the issue of versioning--saving more than one version of a file. All backup services do this. So do Dropbox and Google Drive. But Microsoft's OneDrive does it only with Office documents, such as .docx and .xlsx files.
You don't have to pay for both services. If you use a true backup service for backup, you can probably get by with the limited, free version of your sync service.