SEATTLE (AP) - Several companies lost access to their own files when Amazon.com Inc.'s pay-as-you-go data storage system went down.

Amazon said computers that power its Simple Storage Service were unreachable at one of three data centers for about two hours on Friday. By 7am Pacific Time (3pm GMT), most users' problems were resolved.

The two-year-old storage service is one of several "cloud computing" offerings from Amazon.

Web start-ups and others pay to store and crunch data on Amazon's servers rather than running their own. By the end of 2007, about 330,000 people had registered to use the services.

Simple Storage Service customers flocked to Amazon's support discussion board Friday to report problems, seek updates and vent frustrations.

"S3 service has stopped working about two hours ago. This is really a severe blow to confidence in trusting AWS services," wrote one, under the name Andrea Barbieri.

Several start-ups that use Amazon Web Services, including digital photo sharing site SmugMug Inc. and web e-mail provider Mailtrust, said on Friday they were not affected.

Asheville, North Carolina-based DigitalChalk Inc., which delivers multimedia training over the web, said some of its content was inaccessible as a result of the outage.

"While we are very concerned about the potential impact this had on DigitalChalk.com, we were glad to see that the recovery was fairly rapid and we had no loss of data or files," Tony McCune, DigitalChalk's vice president of sales and marketing, wrote in an e-mail to The Associated Press.

"Our biggest concern going forward will be how well Amazon communicates with their customers about the incident," he said, echoing the online comments of several people affected by the outage.

In an e-mail, Amazon spokesman Drew Herdener wrote, "Any amount of downtime is unacceptable and we won't be satisfied until it's perfect."