Karl asked if cloud-based backup really makes sense when you're making small, frequent changes to a very large file.
That's a good question, and I'm happy to say that the answer is Yes. Let me explain:
It's in the economic interests of cloud-based backup services to upload as little as they can get away with every day. For that reason, they try to avoid backing up very large files. Most of the time, they compare the current version of a large file with the most recent backup, then only upload the changes between the two versions.
To make sure that they do this, I ran some tests using three different cloud-based backup services: Carbonite, IDrive, and Mozy. I created a 100MB TrueCrypt vault and backed it up with all three services. The backups generally took about half an hour. Then I opened the vault (an encrypted file that, when opened, becomes a virtual drive in which you can store your sensitive files), altered a couple of small files, closed the vault, and backed it up again with each service. Each finished this second backup within a minute.
You won't always have such satisfying results. Every so often, these services will decide that they must backup the entire file. But that will probably be the exception, not the rule.
Note: I edited this post about two hours after it went live, removing an error.
Contributing Editor Lincoln Spector writes about technology and cinema. Email your tech questions to him at email@example.com, or post them to a community of helpful folks on the PCW Answer Line forum. Follow Lincoln on Twitter.