SAN FRANCISCO — When people had only one or two computers, file sharing wasn't a big worry.
Now, gaining access to personal files is a chore for people who own an arsenal of computers, smartphones and tablets.
The annoyance of emailing documents to themselves or saving their work to a thumb drive has given new life to an old idea — online storage. People simply save their Word documents, spreadsheets and photos in "the cloud," a Web-based file cabinet accessible from any device that has an Internet connection.
A number of companies focused on online storage are quickly gaining users and attention. New investment is driving a boomlet in the niche business, adding to an already lengthy list of competitors: Dropbox, YouSendIt.com, Cx.com, Box.net, 4Shared and SpiderOak. Apple may do something similar with its iCloud service, to be introduced on Monday.
Google began acclimating people to the notion of storing documents in the cloud with its Google Docs feature in 2005.
And online backup or storage services like MobileMe from Apple, Windows Live SkyDrive from Microsoft, Mozy from EMC and SugarSync are now familiar. What's changed is that more people have discovered a need for them.
Aaron Levie, chief executive of Box.net, an early online storage company based in Palo Alto, Calif., said that the increased adoption of mobile devices and ubiquity of online connections had created a bigger need for companies like his.
Nearly 60 percent of adults with online access own at least two Internet connected devices, according to Forrester Research. Just under 3 percent, or 4.5 million people, have at least nine different gadgets. If that seems to be a lot, think about this: a person may have a home computer and a work computer, and other members of the family may each have computers. Then count smartphones and tablets, and it's not hard to get to a large number of machines.
"It just sort of clicked," Levie. "There ended up being a tremendous amount of interest."
"Our vision is to simplify millions of peoples' lives," said Drew Houston, chief executive of Dropbox, where 25 million users upload files at the rate of 300 million a day. "You don't have to worry that you have some files on your Mac, some stuff on your work computer and then some more on your iPhone."
A growing number of people believe him. Dropbox stores 100 billion files on its servers. Box.net says it has 6 million users while Mozy says it has 3 million.
Meanwhile, storage companies benefit financially from a constant decline in costs as servers and data storage devices get cheaper each year. Leasing server space is five to eight times cheaper than when Box.net started in 2005, Levie said.
The sales pitch for online storage is that it lets users make changes to a Word file, for example, so that there is a single version available from both their work and home computers. It is a process known as synchronization, or sync for short. Users can also collaborate on a documents with colleagues or share video clips and photos with friends.
Many online storage services let users store a minimal amount of data free of charge. For more space, users pay up to $20 a month. (Dropbox gives users who enlist more customers additional storage.) Saved files are accessible from any Internet connected device.
Backing up files is a side benefit. Users no longer risk losing their children's photos if they forget their mobile phone in a cab or their homework if their hard drive crashes.
George Hamilton, an analyst with Yankee Group, said that online storage largely appealed to tech-oriented consumers, although it has been gaining more mainstream adoption recently.