Microsoft has acknowledged an issue tied to the Windows 11 January 2026 security update that can leave some systems unable to boot without manual recovery.
The company says it is still investigating the cause and is collecting reports from both consumers and IT administrators. Microsoft is not advising users to remove the update at this time, stating that the issue appears limited to specific physical devices.
The same January update has also been linked to other regressions. These include problems with Outlook Classic, broken File Explorer customization, sleep mode failures tied to S3, and issues affecting Citrix Director and Remote Desktop.
In a support document first spotted by Windows Latest, Microsoft confirmed it has received reports of devices failing to start after installing the update.
Affected Versions and Update Details
Microsoft has listed two Windows 11 versions as impacted on its release health dashboard.
Windows 11 version 25H2 with KB5074109
Windows 11 version 24H2 with KB5074109
KB5074109 is a mandatory security update that patches close to a hundred known vulnerabilities. Because of its security classification, it installs automatically on most systems.
Users can check whether the January 2026 update is installed by opening Settings, selecting System, then About, and reviewing the OS build number. Systems running build 26200.7623 have the January update installed. Microsoft released KB5074109 on January 13, 2026.
Boot Failure and Error Code
According to Microsoft, affected PCs may fail to boot and display a black screen tied to the stop code UNMOUNTABLE_BOOT_VOLUME.
This error, also known as Stop Code 0xED, is commonly associated with file system corruption, boot configuration problems, or storage-related failures. Microsoft believes the January 2026 Windows 11 update is the trigger in reported cases.
In its documentation, Microsoft states that affected devices show a black screen with a restart prompt. After that point, the system cannot complete startup and requires manual recovery steps.
Windows Recovery Environment usually resolves the issue, though Microsoft notes that recovery may fail in some cases. When that happens, a clean installation using a Windows ISO may be required.
Microsoft also confirmed that reports so far only involve physical hardware. No virtual machines have shown the same behavior.
What This Means for Users
Microsoft has not disclosed how many systems are affected or which hardware configurations are most at risk. The company has already issued an emergency update, KB5078127, to address other problems introduced by the January patch.
For users whose systems are running normally, no action is recommended. Windows runs across billions of devices, and issues like this often surface under narrow hardware conditions. If a system has shown unusual behavior or failed to boot since mid-January, KB5074109 is the likely cause.
Source: Windows Latest
