NEW YORK (Reuters) - Users of Microsoft Corp’s Sidekick mobile phone may have permanently lost data such as contacts, photos and calendar entries due to the failure of a Microsoft server computer.

Models Ashleah O'Shea (L) and Arielle Vandenberg take a pictures of themselves at the party for the launch of T-Mobile Sidekick 3 in Los Angeles June 20, 2006. REUTERS/Mario Anzuoni

T-Mobile USA, a unit of Deutsche Telekom, said in a notice available on its website on Monday that its customers who do have such data stored locally on their Sidekick devices will “almost certainly” have lost the data.

It also advised customers against resetting a Sidekick by removing the battery or letting the battery drain as this would still result in the loss of any personal content stored on the device made by Danger, a company Microsoft bought in 2008.

When operating normally, the Sidekick retrieves data from Microsoft servers after an event such as a phone reset.

“Personal information stored on your device such as contacts, calendar entries, to-do lists or photos that is no longer on your Sidekick almost certainly has been lost as a result of a server failure at Microsoft/Danger,” a Microsoft unit, the company said.

T-Mobile USA said in the statement dated October 10 that it would update customers again on Monday. T-Mobile USA spokesperson Cara Walker noted that only a minority of the company’s Sidekick customers have lost data due to the outage.

But Walker said that T-Mobile USA was still advising customers not to reset their devices on Monday.

Microsoft said in an emailed statement that the recovery process has been “incredibly complex” because it suffered a confluence of errors from a server failure that hurt its main and backup databases supporting Sidekick users.

The glitch comes as technology companies are increasingly looking to convince customers to use remote storage services to back up their data and at a time when Microsoft is fighting to regain ground lost in smartphones.

Because Sidekick runs on a proprietary Danger system Microsoft said that its other computing services were totally separate and did not rely on Danger’s technology.

T-Mobile USA, the No. 4 U.S. mobile service, said that while its workers were seeking a way to recover the information, “the likelihood of a successful outcome is extremely low.”

The operator, which has been losing customers to both bigger and smaller rivals, said it is currently focused on looking for a way to restore data and is also considering additional measures for customers who have lost data.

Two Sidekick phones were listed as “temporarily out of stock” on T-Mobile USA’s website on Monday.

Microsoft shares were up 3 cents at $25.59 on Nasdaq.