Several days after Amazon.com's cloud outage knocked some high-profile Web sites offline, the company said its cloud service was largely back up and running. Now Amazon is trying to track down the root of the problem.

The outage partially disabled or knocked out popular websites including Quora, Foursquare and Reddit.

On Saturday, two days after Amazon suffered a failure in its Web hosting services, the company announced that it had fixed most of the problem. However, the latest update on Amazon's Service Health Dashboard noted that engineers are still working on some remaining issues with its EBS, or Elastic Block Storage.

At 10:35 p.m. ET on Sunday, Amazon reported, "We're in the process of contacting a limited number of customers who have EBS volumes that have not yet recovered and will continue to work hard on restoring these remaining volumes.

Users still having problems with their hosted Web sites should contact Amazon on their Web Services site. Users should select Amazon Elastic Compute Cloud in the "Services" field. And in the description field, they should list the instance and volume IDs and describe the issue they're experiencing.

The company also noted on its dashboard that workers are "digging deeply into the root causes of this event" and will post their findings in a post mortem.

The trouble started a little after 5 a.m. Eastern on Thursday when the company's Service Health Dashboard reported connectivity problems that were affecting its Relational Database Service, which is used to manage a relational database in the cloud, across multiple zones in the eastern U.S.

Because of server problems at Amazon's data center, which handles the company's EC2 Web hosting services, some websites, including popular Web 2.0 sites, were left staggering or disabled.

Web sites Reddit, Foursquare, Quora and HootSuite, which suffered through Amazon's outage, are back up today.

Sharon Gaudin covers the Internet and Web 2.0, emerging technologies, and desktop and laptop chips for Computerworld. Follow Sharon on Twitter at @sgaudin or subscribe to Sharon's RSS feed . Her e-mail address is sgaudin@computerworld.com.