No matter how well Bitcasa guesses what you need, some files must be fetched from the cloud before they can be accessed. Because not everyone has a perfect connection, some users might not get a seamless experience. And if a computer is offline, some files will simply be unavailable.

Inviting people to store "infinite" data requires Bitcasa to provide a lot of storage space. The company plans to keep the expense of all that storage to a minimum by tracking if a file it needs to store for one user is an exact copy of something already being stored in the cloud for someone else—a popular music track, for example. In such situations, Bitcasa needs to store only one copy, Gauda says.

The notion of cloud storage seamlessly extending a computer's hard drive has already gained considerable popularity in the form of Dropbox. A Dropbox folder on a user's computer is always mirrored in the cloud, as well as on a user's other computers. The company provides two gigabytes of storage for free, and 50 gigabytes of storage for $10 a month. Bitcasa says it will offer unlimited amounts for the same price, but the new company might need to introduce premium options, perhaps for businesses, if users chew up large amounts of capacity. Cloud backup service Mozy had to withdraw its "unlimited" storage option earlier this year because it cost too much to maintain.

After seeing Bitcasa's demonstration at the Disrupt event, venture capitalist Roelhof Botha of Sequoia Capital, which has invested in Dropbox, noted that storing and transporting so much data would be a challenge. "It's going to be expensive to stream all that data," he said.

Angel investor Ron Conway said he expected the startup to face stiff competition from Dropbox, now valued at over $1 billion. Conway was one of Dropbox's early stage investors. Conway added, however, that Bitcasa has already shown off technology that could set it apart. "The thing that caught my eye was the streaming video," he said. "That seems really difficult for other services to do."