Microsoft Confirms Windows 11 24H2/25H2 Bug Blocks Access to the System Drive C

Microsoft is investigating a critical post-update issue (KB5077181, OS Build 26100.7840) that causes the system drive (C:) to become inaccessible on certain Samsung Windows 11 consumer devices (notably Galaxy Book 4), displaying “C:\ is not accessible – Access denied.” The failure blocks applications, elevation, and remediation, is reported across multiple countries, is believed to involve corrupted ACLs potentially linked to Samsung Share, and as of March 13, 2026 no official fix or safe workaround has been released.