About one-third of those who use Microsoft’s MSN Messenger service--about 6 million people--lost the ability to log on this week or were denied access to their “buddy lists,” and many are still waiting for their service to be restored.

According to a Microsoft spokesman, the glitch was a result of “an extremely rare set of circumstances that occurred when our database servers had a disk controller fail and the backup for this controller also had an error occur, which resulted in us not being able to fix the issue immediately.”

Some users have been without service since Tuesday evening.

MSN Vice President Rick Bray indicated the company has identified the issue that caused the service interruption, which necessitates rebooting and restoring all affected MSN Messenger servers.

“We are in the process of restoring the backup data for the remaining customers who are still experiencing the service outage,” Bray said in a statement. “We expect to fully restore service sometime later today. Until then, some customers may continue to experience an inability to log into MSN Messenger.”

Billy Pidgeon, an analyst for Jupiter, said that service outages happen “quite often” but generally last for hours, not days. He said this particular outage became a flash point for Microsoft because it involved buddy lists, which most users consider sacred.

“People find it disturbing when they lose their buddy lists,” he said.

Bray attempted to assuage those fears.

“We want to assure our customers that their buddy lists have not been lost," he said.

Also disturbing is that the servers weren’t backed up adequately, Pidgeon said.

“They’ll have more redundancy in the future, I’m certain of that,” he said.

However, Microsoft said it has additional backup systems that let it fully restore service, but this system is “more time consuming.”

About 18.4 million users have downloaded the MSN Messenger software and are using it at least once per week, according to industry research firm Jupiter Media Metrix. This compares to industry leader AOL, which has about 25.5 million users, and Yahoo, which has 11.7 million users, said Jupiter.

In February, Microsoft also encountered problems with the database servers that manage MSN Messenger, some of which were caused by hacker attacks. Pidgeon thinks suffering a second outage so soon will hinder Microsoft’s effort to close the gap between it and industry leader AOL.

“This is not good for Microsoft, because they are not the leader in this particular space, and they would like to be,” he explained. “This doesn’t instill any confidence on the part of the users.”