Hard drives are constantly getting bigger, but it seems we fill them up with data just as quickly anyway. The solution may be a small piece of software from a startup called Bitcasa; when you install it, the capacity of your computer's hard drive becomes, essentially, infinite.

"Look at the bottom of my finder [file browser] window," says Tony Gauda, one of the cofounders of the company, a finalist in a competition for new startups at the Disrupt conference in San Francisco this week. "It says 18 terabytes available, because Apple's operating system doesn't understand numbers any bigger, but I can actually save infinite data if I want to."

Bitcasa is currently in a limited beta; users pay $10 a month for unlimited storage. The software only works on Apple computers, but a Windows version is in development, and mobile apps for smart phones and tablets are planned, too. "We want you to be able to have one infinitely large store of your files and data that you can access from any device," says Gauda. "We will be the last storage device or service you ever buy."

Bitcasa creates the illusion that data stored on distant cloud servers is actually kept on the physical hard drive inside a machine. When that computer's user opens a file browser window, all files and folders appear to be stored locally.

As a file stored in Bitcasa's cloud is opened, it is downloaded to the computer as quickly as possible. "The network is good enough that it can be your hard drive now," says Gauda. "It performs well enough that you won't notice that we're streaming a movie to you rather than playing it off your hard drive."

Bitcasa's software shuffles a user's data around to ensure that the most-used files are stored locally, on a computer's built-in hard drive, to minimize the effects of slow or missing connections. "We analyze how you use your data by looking at things like how old a file is, or when you last used it, to decide what gets put into the cloud, but you can also specify folders that should always be stored locally," Gauda explains. Holiday snaps from a couple of years ago, for example, would reside on Bitcasa's servers. A document created last week is more likely to be stored on the computer's hard drive.