Microsoft has officially acknowledged a critical bug affecting Windows 11 users on certain Samsung devices, in which the system drive (C:) becomes completely inaccessible after installing the February 2026 security update. The company is now actively investigating the issue in coordination with Samsung.

After installing the February 2026 Patch Tuesday update (KB5077181, OS Build 26100.7840), affected Samsung devices display the error message: “C:\ is not accessible – Access denied.”

The error effectively locks users out of their own system drive, blocking the launch of applications, including Microsoft Outlook, Office apps, web browsers, system utilities, and Quick Assist.

The failure is not triggered by any specialized operation. Routine tasks such as opening files, launching applications, or performing standard administrative actions are enough to surface the error.

In severe cases, users are also unable to elevate privileges, uninstall updates, or collect diagnostic logs due to cascading permission failures, making self-remediation nearly impossible without external support.

Microsoft confirmed the issue is primarily observed on Samsung Galaxy Book 4 and other Samsung consumer devices. Reports have emerged from multiple regions, including Brazil, Portugal, South Korea, and India. The affected Windows versions are Windows 11 version 25H2 and Windows 11 version 24H2 no server platforms are currently impacted.

Enterprise administrators have also flagged the problem on Reddit’s r/sysadmin community, noting that affected Galaxy Book laptops within Active Directory domains cannot have NTFS permissions modified, even using administrator credentials.

Secondary failures reported in the field include trackpad driver failures and the inability to open PowerShell, as its binary resides on the now-inaccessible C: drive.

Microsoft’s latest investigation points to the Samsung Share application as a probable contributing factor, though the root cause has not yet been fully validated.

The company stated it is working with Samsung to determine whether the fault lies with the Windows update itself or with Samsung’s software ecosystem on the affected devices. The issue is believed to involve corrupted or incorrectly applied Access Control Lists (ACLs) on the root of the system drive.

As of March 13, 2026, Microsoft has classified the issue as “Investigating” and has not yet released an official fix or workaround. A Reddit user claiming to be a Samsung technician in Brazil posted an unofficial workaround involving reassigning C: drive ownership to the “Everyone” group, but Microsoft security experts strongly caution against this approach, as it strips Windows’ built-in security protections from critical system directories.

Microsoft has advised affected users to wait for an official patch and has committed to providing further updates as the investigation progresses.

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