Three days after it temporarily halted the availability of the Windows RT 8.1 update from Windows Store, Microsoft has restored the download. According to the firm, a very small percentage of Windows RT upgraders experienced issues with the original version of the update.

I wrote about the original issue in Windows RT 8.1 Update Temporarily Unavailable. At the time, Microsoft noted that it was investigating a situation affecting a limited number of users updating their Windows RT devices to Windows RT 8.1. "We have temporarily removed the Windows RT 8.1 update from the Windows Store," a Microsoft representative told me Saturday. "We are working to resolve the situation as quickly as possible and apologize for any inconvenience."

On Monday, the firm provided the following update:

"Based on our investigations of a situation customers have encountered updating to Windows RT 8.1, we can confirm that as of now this is a Windows update issue only affecting Surface RT customers. While only less than 1 out of every 1,000 (or less than 0.1%) Surface RT customers who have installed Windows RT 8.1 have been impacted, improving their experience and ensuring their systems are fully operable as quickly as possible is our number one priority. We have made recovery media available for download along with actionable guidance for affected customers. We continue to work towards making the Windows RT 8.1 update available in the Windows Store again and apologize for any inconvenience. Further updates will be provided as they become available."

Today, the update has returned, and those still running Windows RT 8.0 can update to the latest version of the OS normally, by visiting the Windows Store and following my step-by-step upgrade instructions. Microsoft has provided the following explanation of the issue:

"This was due to a rare situation where firmware updates had not completed at the time of the update to RT 8.1. In most cases, if a customer encountered this issue the result was simply an extra reboot. However, for a very small percentage, the boot configuration data was affected which prevented a successful boot. We worked to quickly resolve the issue and now encourage customers to update their Surface RT devices. Surface Pro and 8.1 customers were not impacted by this issue.

As previously announced, there is a recovery solution available for download for Surface RT customers who experienced the installation issue. We apologize for any inconvenience this may have caused. If an issue occurs with our software or devices, we take immediate steps to ensure a quality experience for every single customer—that's been our driving priority in this case, despite the very limited number of customers impacted. We are grateful to the customers who shared their Surface RT devices, assisting our efforts to determine the root cause quickly."

It's worth noting, too, that any user who successfully completed an upgrade from Windows RT to Windows RT 8.1 is all set: There's no problem with your install and nothing left to do.

Shine on.