As has been mentioned this idea is quite old: Freenet, OceanStore.
The biggest problem of your approach is the economics: convincing someone to give up storage in order to receive storage. "Samsara" is perhaps the closes project to this idea. In this system, one client gives up a block for storage by another client and gets a "rain-check" to use a block on the former's machine. The project was not well received by the research community b/c the economics were not convincing.
Security is an issue, but can be solved with appropriate algorithms.
Storage is cheap, perhaps the gmail approach with some privacy modifications is the best current public backup storage option.
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As has been mentioned this idea is quite old: Freenet, OceanStore. The biggest problem of your approach is the economics: convincing someone to give up storage in order to receive storage. "Samsara" is perhaps the closes project to this idea. In this system, one client gives up a block for storage by another client and gets a "rain-check" to use a block on the former's machine. The project was not well received by the research community b/c the economics were not convincing. Security is an issue, but can be solved with appropriate algorithms. Storage is cheap, perhaps the gmail approach with some privacy modifications is the best current public backup storage option.