T-Mobile and Microsoft said on Saturday that personal information that had disappeared from some customers' Sidekick mobile devices was almost certainly lost and apologized for the incident.

Affected data includes contacts, calendar entries, photos, notes, and saved e-mails.

T-Mobile has partnered with Microsoft subsidiary Danger to manage data services for its Sidekick mobile devices. Danger began experiencing technical troubles last week that lead to the loss of some of the customer data stored on its servers.

For Sidekick users who ran out of battery power, removed their battery, or reset their devices during this period, data was lost both locally and remotely.

The company said that it plans to publish an incident update on Monday.

T-Mobile is warning users not to remove their Sidekick batteries or reset their devices because "the Microsoft/Danger network remains unstable and all content (including contacts, calendar, and notes) that you re-input into your device is not backed up on the network."

T-Mobile said that it will credit affected customers the cost of their data plan for one month.

Those affected are not happy and have been complaining on T-Mobile's online forum and elsewhere.

"I've been with T-Mobile for almost five years," said a forum post attributed to "Lanisa." "I want to leave them after this fiasco! All my important information is now lost."

A widely circulated post on Sidekick news site Hiptop3.com claims that Microsoft hired Hitachi to upgrade its Storage Area Network (SAN) and that no backups were made before the upgrade process began. When problems arose during the upgrade, data was lost.

At the time this story was filed, Hiptop3.com was overloaded with traffic and non-responsive. An excerpt from Hiptop3.com's report has been posted to T-Mobile's forum.

Declining to comment on that report, a Microsoft spokesperson said, "Microsoft is working around the clock to understand the situation."

John Pescatore, VP and research fellow in Gartner Research, said the incident was pretty unusual, noting that there have been plenty of cloud computing outages but nothing comparable in terms of data loss that he could recall. He said that while he had no confirmation about the report of the botched upgrade, it's "Business 101 that you make a backup."

To Pescatore, the incident represents a cautionary tale about risk that businesses face if they use consumer Internet services. "The consumer-grade services by definition are not business ready," he said. "The issue is not that cloud computing can't be done securely. It's that consumer clouds don't meet enterprise needs."

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