Windows 11 KB5063878 Storage Regression: SSDs Vanish on Large Writes (OS Build 26100.4946)

A mid‑August cumulative update for Windows 11 — KB5063878 (OS Build 26100.4946), released on August 12, 2025 — is being linked by multiple independent community tests and tech outlets to a storage regression that can render some SSDs inaccessible after sustained, large sequential writes, with affected drives reportedly disappearing from the OS and, in some cases, exposing file corruption and unreadable SMART data. (support.microsoft.com)

Background / Overview​

Microsoft released KB5063878 on August 12, 2025 as the August cumulative quality and security rollup for Windows 11 (24H2). The official update notes list security fixes and assorted quality improvements and — at the time of publication — do not list a storage‑device failure as a known issue. Microsoft did, however, acknowledge and issue a mitigation for a separate enterprise installation problem (error 0x80240069) that blocked the update when deployed via WSUS/SCCM. (support.microsoft.com, bleepingcomputer.com)
Within days of the rollout, community test reports and multiple tech sites began compiling reproducible symptoms: during sustained writes of roughly 50 GB or more, certain NVMe SSDs (and a small number of HDDs in a few reports) can stop responding, vanish from Device Manager and Disk Management, and present unreadable SMART/controller data to host tools. Reboots sometimes restore temporary visibility but do not guarantee file integrity. These accounts were aggregated and discussed in enthusiast forums and on social platforms, producing model lists and reproduction notes. (notebookcheck.net, guru3d.com)
This pattern is technically reminiscent of prior incidents where changes to the host/OS interactions with NVMe Host Memory Buffer (HMB), kernel buffering, or driver timing exposed latent firmware bugs in some controllers. The current cluster of reports centers on drives using Phison controller families and several otherwise popular branded SSDs, though the symptom set is not strictly limited to a single controller vendor. The community analysis remains a working hypothesis pending vendor and Microsoft telemetry confirmation. (igorslab.de)

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What users are reporting​

Symptoms (consistent community fingerprint)​

• The drive becomes unresponsive or “disappears” from Windows while a large, continuous file transfer is in progress.
• SMART attributes and controller information become unreadable by software tools.
• A system reboot can restore the drive’s visibility temporarily, but repeated heavy writes may reproduce the failure.
• Files written during the affected window are at risk of being corrupted; some users report missing partitions after a reboot. (notebookcheck.net)

Typical trigger and reproducibility​

Community testers and a public thread of technical reproductions converge on a typical trigger window: sustained sequential writes on the order of ~50 GB or more, with controller utilization climbing and certain buffering paths exercised for multiple seconds or minutes. That workload profile — common during large game updates, bulk media transfers, or disk cloning — appears to be where the regression surfaces. Reproducibility varies by controller firmware, motherboard firmware (UEFI), and system configuration. (notebookcheck.net)

Affected models and controller families (community‑compiled lists)​

Community collations and independent testing have frequently highlighted these models/controller families (lists are community‑generated and not definitive):

• Corsair Force MP600 (Phison E16 / related variants)
• SSDs built around the Phison PS5012‑E12 and E21T families
• Kioxia Exceria Plus G4 (Phison‑based SKUs)
• SanDisk Extreme Pro M.2 NVMe 3D (reported in some tests)
• Third‑party branded models such as Fikwot FN955 and others that use Phison or similar silicon

Important caveat: lists vary between testers; firmware revision and host platform materially affect whether the fault manifests. Not every Phison‑based SSD is affected, and several non‑Phison drives have also shown trouble in isolated reports. Treat model lists as early indicators, not definitive recall lists. (guru3d.com, igorslab.de)

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Technical analysis — plausible causes and mechanics​

Why sustained writes matter​

Large sequential writes stress multiple layers of the storage stack simultaneously: application‑level buffers, OS page cache, kernel I/O scheduling, driver queues, the NVMe command stream, the SSD controller’s DRAM (or lack thereof), and flash translation metadata updates. A subtle regression in the OS’s buffering or NVMe driver timing — or a host change that increases HMB allocation or alters command ordering — can push controller firmware into an edge case it previously avoided. When a controller “locks up” or stops responding to admin commands, the OS may treat the device as removed from the PCIe topology. The result is the device vanishing from Device Manager and SMART data becoming unreadable.

Host Memory Buffer (HMB) and DRAM‑less controllers​

Many low‑cost NVMe designs are DRAM‑less and rely on the NVMe Host Memory Buffer (HMB) mechanism to use a slice of host RAM for caching FTL metadata. HMB behavior ties drive performance and stability to host memory allocation patterns and driver implementations. Past incidents with Windows 11 updates have shown that changes to HMB allocation size or timing can expose latent firmware bugs in some controllers. Community analyses of KB5063878 point to a similar host/driver interaction hypothesis, though direct vendor telemetry tying KB5063878 code paths to the failure remains unpublished. This makes the HMB hypothesis plausible but not yet formally confirmed by vendors or Microsoft. (igorslab.de)

Kernel/driver regression vs. firmware defect​

Two non‑exclusive root hypotheses explain how an OS update can cause these symptoms:

• A kernel/driver regression changes command ordering, timing, or resource allocation and triggers a latent firmware bug that previously sat dormant.
• A firmware edge case is exposed by revised host behavior (for example, larger or more frequent HMB allocations), revealing a controller flaw that requires a firmware patch.

Community tests and forum breakdowns lean toward a host‑behavior trigger exposing a controller‑level weakness, but without vendor logs or Microsoft‑side debug traces, the causal chain cannot be closed. This nuance matters for remediation because fixes can require either an OS update, a firmware update, or both.

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What vendors and Microsoft have (and haven’t) said​

• Microsoft: the KB5063878 support page documents the update and its release date (August 12, 2025) and initially lists no known storage issues. Microsoft separately acknowledged and mitigated the WSUS/SCCM deployment failure (error 0x80240069) and issued a fix/servicing control for that distribution regression. There is no published Microsoft Knowledge Base entry explicitly acknowledging a confirmed SSD disappearance bug tied to KB5063878 at the time of reporting. (support.microsoft.com, bleepingcomputer.com)
• SSD manufacturers: as of the publishing of this article, major vendors named in community lists had not posted consolidated bulletins directly attributing a KB5063878‑triggered failure across product lines. Several tech outlets and community aggregators explicitly note the absence of official vendor confirmation and caution against treating community model lists as definitive. No vendor‑issued firmware advisories that universally map to KB5063878 are publicly recorded at the time of writing. (igorslab.de, notebookcheck.net)

Because neither Microsoft nor the drive makers have published a universal root‑cause statement tying KB5063878 definitively to permanent drive failure, community findings must be treated as high‑risk warnings rather than settled facts. That said, consistent reproducible symptomology across independent testers elevates the issue from rumor to credible incident that requires vendor investigation.

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Practical guidance — what to do now (consumers)​

The following recommendations consolidate community mitigations, vendor best practices, and Microsoft’s public notes. Cross‑reference with vendor tools and official updates before taking irreversible steps.

• Back up critical data immediately to an independent device or cloud service. Backups are the only reliable defense against drive‑level corruption.
• If you have not installed KB5063878 yet and your workflow includes large sequential writes (game installs, bulk media transfers, disk cloning, video exports), consider delaying the update until vendors or Microsoft publish mitigation/firmware guidance. (notebookcheck.net, wccftech.com)
• If KB5063878 is already installed:
• Avoid performing large continuous writes (> ~50 GB) to NVMe drives that may be Phison‑based or DRAM‑less until further notice.
• Monitor drive health with vendor utilities (Corsair iCUE, SanDisk Dashboard, Kioxia Storage Utilities), CrystalDiskInfo, or smartctl and capture logs if anomalies appear.
• If you observe a drive vanish during a transfer, stop further writes and take a forensic image if the data is critical; avoid repeated reboots that may complicate recovery. Community guidance recommends preserving the drive’s state for imaging where possible.
• If you administratively control updates, use Windows Update controls to defer or block the KB while waiting for firm vendor/Microsoft guidance. For home users, pause updates via Settings → Windows Update → Pause updates. For enterprises, apply WSUS/SCCM controls. Microsoft has used Known Issue Rollback and servicing controls in similar rollouts — keep an eye on the Windows release health dashboard for formal mitigations. (bleepingcomputer.com, support.microsoft.com)

Registry and emergency mitigations — handle with caution​

Some experienced users have shared registry or driver‑level mitigations (for example, limiting or disabling HMB allocations via storport/stornvme parameters) that were used in prior HMB‑related incidents. These are emergency workarounds that can reduce performance and carry risk. Only advanced users who understand the implications should consider them, and always back up before editing the registry. Community commentary emphasizes that disabling HMB can be a temporary stopgap but may not resolve every manifestation of the fault.

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For IT administrators and procurement teams​

• Inventory endpoints: map drive models, controller families, and firmware revisions. Prioritize systems with DRAM‑less NVMe drives and Phison controllers for closer monitoring.
• Hold fleetwide installation of KB5063878 on at‑risk hardware pending vendor/Microsoft guidance; use WSUS/SCCM approval controls. Microsoft’s initial Known Issue Rollback for the WSUS installation error is an example of the company using servicing controls to limit impact for enterprises. (bleepingcomputer.com)
• Require firmware validation before upgrading: coordinate with SSD vendors to identify firmware revisions explicitly tested against the Windows 11 24H2 update. Firmware updates can fix controller issues but carry flash/update risk; test in a lab and back up data before mass flashing.
• If drives fail during imaging or provisioning tasks, treat the symptom as a potential data‑integrity event and escalate to vendor support with logs, host traces, and a detailed reproduction recipe.

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How to check if KB5063878 is installed (concise steps)​

• Press Win + R, type winver, and press Enter. The About/WinVer dialog shows the OS build. KB5063878 corresponds to OS Build 26100.4946.
• Alternatively, open Settings → Windows Update → Update history to view installed updates.
• On managed networks, check WSUS/SCCM deployment logs for the package name or consult your update catalog. (support.microsoft.com)

If you need to remove the cumulative LCU portion of the combined SSU+LCU package, follow Microsoft’s documented DISM removal steps for the LCU package name; be aware that the combined package includes the servicing stack (SSU), which cannot be removed with wusa.exe uninstall of the combined package. Carefully follow Microsoft’s published guidance before attempting removal. (support.microsoft.com)

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What to do if your drive has already stopped responding​

• Immediately stop writing to the affected machine and disconnect the drive if feasible. Continued operations risk overwriting recoverable sectors.
• Capture logs and diagnostics: vendor dashboard logs, smartctl outputs, and Windows Event Logs. This information is often essential for vendor investigation and data‑recovery vendors.
• If the data is critical, consider powering off and removing the drive for forensic imaging by a recovery specialist rather than attempting repeated reboots or repairs that could further damage recoverable data. Community guidance strongly recommends imaging the device when data integrity is at stake.
• If you decide to attempt in‑place recovery, use vendor tools to check firmware and run diagnostics; only proceed to reformat or reinitialize when you have a verified backup or no other recovery options remain.
• Contact your SSD vendor’s support channel and supply diagnostics and reproduction steps. If a large‑scale or reproducible failure is confirmed, vendors typically coordinate firmware advisories and remediation paths.

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Strengths of the current evidence — and the gaps​

• Strengths:
• Multiple independent community tests report a consistent symptom profile anchored around large sequential writes (~50 GB) and drive disappearance. These tests include methodical reproductions and model‑level correlations. (notebookcheck.net, guru3d.com)
• Several reputable tech outlets have aggregated and reported the community findings, increasing visibility and cross‑checking initial claims across jurisdictions. (wccftech.com, igorslab.de)
• Microsoft responded quickly to the enterprise installation regression (0x80240069), demonstrating active servicing controls were applied to at least one KB‑related distribution problem. (bleepingcomputer.com)
• Gaps / risks:
• No universal vendor or Microsoft bulletin (as of this writing) conclusively ties KB5063878 to a specific firmware bug that would explain every reported failure. That absence leaves room for multiple co‑factors (firmware revision, motherboard BIOS, driver states) to be involved. Treat community model lists as indicatory, not exhaustive.
• The exact causal chain — OS change vs. firmware latent bug — is not closed without vendor telemetry and coordinated debug traces. This matters because an OS patch, device firmware update, or both may be required to fully resolve the fault.
• The volume of public reports, while serious, has not (yet) risen to the scale of a global recall; incidence may be concentrated in specific controller firmware states or platform combinations.

Given these strengths and gaps, the precautionary principle applies: if you rely on at‑risk drives for critical data, minimize the chance of exposure and follow the practical mitigation steps above while awaiting vendor/Microsoft confirmation.

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Final assessment and next steps​

The KB5063878 story is an important reminder that complex interactions between modern OSes and diversified storage silicon can produce pernicious edge cases. The symptoms reported — drives vanishing under sustained writes and potential file corruption — are severe and have meaningful consequences for both consumers and enterprise fleets. The independent community reproductions and aggregated reporting by multiple outlets make the anomaly credible and actionable: back up, avoid heavy writes, and delay the update on vulnerable systems until vendors and Microsoft publish clear remediation guidance. (notebookcheck.net, igorslab.de)
For now, key practical next steps are:

• Prioritize backups and avoid bulk transfers on SSDs that are suspected to be Phison‑based or DRAM‑less. (wccftech.com)
• Administrators should hold KB deployment on exposed fleets and coordinate firmware validation with SSD vendors. (bleepingcomputer.com)
• Monitor vendor dashboards and Microsoft’s Windows release health page for formal advisories and firmware/OS patches. (support.microsoft.com)

Finally, treat any current model lists and reproduction notes as investigative leads. They are valuable for triage and risk reduction, but definitive attribution and a permanent fix will depend on coordinated vendor telemetry, firmware updates, or an OS servicing change. Where claims cannot be corroborated by manufacturer telemetry or an official Microsoft statement, those claims are flagged as plausible but not yet verified and should be handled with appropriate caution.

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Conclusion
The KB5063878 rollout underscores an uncomfortable truth for modern PC users: updates that improve security can sometimes interact badly with heterogeneous hardware in the wild. The evidence compiled so far points to a reproducible and high‑risk storage regression tied to heavy sequential writes on some NVMe drives, often those using Phison‑family controllers. Until manufacturers or Microsoft publish a coordinated, tested fix, the safest posture is to preserve backups, avoid large write workloads on suspect drives, and defer the KB for systems that cannot tolerate the risk. The situation remains fluid — expect vendor firmware advisories or a Microsoft servicing update, and track official channels for verified remediation. (support.microsoft.com, notebookcheck.net)

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Source: Cyberockk Windows 11 Update KB5063878 Causes SSD Problems After Big Data Transfers