It's "one file" in the sense that you're backing up all your important stuff with one file upload.
You can backup as many files as you'd like, but if you name them the same thing on upload, they will rewrite each other. I like to name my backups by date, so I end up with files named 20121009.backup.tar.gz.enc. Tomorrow it will be 20121010.backup.tar.gz.enc.
It is preferred that people encrypt their backups before upload, but it's not enforced. There is no checking of file contents.
Why is this service free?
Because I like to develop software and this is my contribution to the opensource community. There are plans to provide a paid service to provide revenue that will keep the service sustainable. As it stands, there are multiple tebibytes of storage available, so tarbackup should be available for free well into the future.
I think a pertinent question to answer would be "How is this service free"?
Great question. It's "free forever if you sign up now"
This project started out as fun, but after some feedback that "backup can't be free", I've planned out pricing to sustain the service long-term.
If you sign up before the switch, the service will continue to be free and unlimited forever because I appreciate your support. At some point in the future, the service will be priced at $0.07/GB/mo; 20% less expensive than raw S3 storage and 75% less expensive than tarsnap.
This pricing will pay for all expenses and allow the service to grow and pay for maintenance.
Uh oh, if you think it's "too good to be true" the price is probably too low! We'll see; if people don't sign up at $0.07, I guess I'll just raise the price :P (marketing is funny). I've been having a blast making the service and seeing people use it, so thank for your interest and support.
It sounds really interesting, but it still doesn't answer the question of how you are able to offer this service at that price? Is there something we don't know?
$0.07 / GB was calculated from costs and covers all expenses for an offsite backup service like this. I'm an open book and the project is open-source, so if you have any questions, feel free to ask. :)
It is supposed to receive encrypted files anyway, so it does not check its contents.