Microsoft is having trouble updating machines to Windows RT 8.1. The company has taken the update offline, preventing people from upgrading their RT devices. In a statement published on Microsoft's support forums, the company says it is "investigating a situation affecting a limited number of users updating their Windows RT devices to Windows RT 8.1. As a result, we have temporarily removed the Windows RT 8.1 update from the Windows Store." Microsoft hasn't specified what the issue is, but a number of reports suggest that a problem that has caused some machines to have their boot data damaged during the update is to blame. Affected machines are unusable until repaired using a recovery drive. There's no word how long the update will be offline, but in the statement Microsoft says it "will provide updates as they become available."