Windows 11 24H2 KB5063878 SSD Issue: Backups and Mitigations

Microsoft and several SSD vendors are investigating reports that the August 12, 2025 cumulative update for Windows 11 24H2 (KB5063878, OS Build 26100.4946) can trigger a reproducible storage regression where some SSDs vanish from the operating system during sustained, large sequential writes — a failure profile that in some community tests has produced truncated or corrupted files and, in a minority of cases, drives that remain inaccessible after reboot.

Background / Overview​

Microsoft shipped KB5063878 as the August 12, 2025 combined servicing stack update (SSU) and latest cumulative update (LCU) for Windows 11 version 24H2 (OS Build 26100.4946). The official Microsoft support page lists the package contents and, at initial publication, stated that Microsoft was “not currently aware of any issues with this update.”
Within days of the public rollout, multiple independent community testers and specialist outlets reproduced a consistent failure fingerprint: during sustained, large sequential write workloads — commonly reported by testers near the ~50 GB mark and more likely when target drives were already substantially used — the target drive could stop responding, disappear from File Explorer, Device Manager and Disk Management, and return unreadable SMART/controller telemetry. Reboots sometimes restored visibility, but files written during the event were often corrupted or incomplete; in some reports the drive did not reappear without vendor-level intervention.
These accounts have been amplified by enthusiast sites and aggregated community threads, and the issue has drawn vendor attention — notably from Phison, a major SSD-controller supplier, which publicly acknowledged that it was investigating possible industry-wide effects linked to KB5063878 (and a related preview update KB5062660). Microsoft has told reporters it is actively working with storage partners to reproduce and diagnose the reports and has asked affected customers to provide additional telemetry and Feedback Hub submissions. (bleepingcomputer.com, tech.yahoo.com)

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What the reports actually show​

Symptom profile (consistent across many hands‑on tests)​

• A large, sustained sequential write begins normally (examples: large game update, archive extraction, disk cloning) and then abruptly fails after tens of gigabytes are written — community tests commonly observe failure after roughly 50 GB of continuous writes.
• The target drive becomes unresponsive and can vanish from the OS topology (File Explorer, Device Manager, Disk Management). Vendor utilities and SMART telemetry may become unreadable.
• Rebooting sometimes restores the device to the OS, but written data in-flight is frequently truncated or corrupted. In a minority of tests the drive remained inaccessible and required vendor or forensic-level work.

Typical trigger and exposure factors​

Community reproducibility suggests a narrow workload window rather than random hardware failure:

• Sustained, sequential writes on the order of tens of gigabytes (examples: game installations, extraction of large archives, cloning operations).
• Drives already substantially used (community reports commonly cite >50–60% used capacity) — a condition that reduces spare area and SLC cache effectiveness.
• An over‑representation in early lists for Phison-based controllers and DRAM‑less SSDs, although subsequent tests implicated a variety of controllers and brands; this means the root cause is likely an interaction between host behavior and controller firmware rather than an isolated brand defect. (tomshardware.com, guru3d.com)

Important caveat: community model lists are investigative leads, not definitive blacklists. Firmware revision, host chipset, BIOS settings, NVMe options and even motherboard firmware can materially alter whether a particular drive reproduces the fault. Treat early model collations as signals, not absolute truth.

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Who and what vendors have said so far​

• Microsoft: the KB entry for KB5063878 confirms the release details but initially listed no known issues; after industry reporting, Microsoft told at least one outlet it could not reproduce the issue in its internal testing on up-to-date Windows 11 24H2 systems and asked affected customers to submit reports to the Feedback Hub and to Microsoft Support so the company could collect additional diagnostic data. Microsoft also emphasized it is working with storage partners to reproduce the problem. (support.microsoft.com, bleepingcomputer.com)
• Phison: publicly acknowledged it had been “recently made aware of the industry‑wide effects” tied to KB5063878 and KB5062660 and said it was reviewing controller families that may have been affected while collaborating with partners. Phison later moved to counter a circulating, falsified internal document that claimed to list impacted controllers, taking legal steps against the leak while continuing the investigation. (tech.yahoo.com, tomshardware.com)
• Specialist outlets and hobbyist test benches (Tom’s Hardware, Guru3D, NotebookCheck and others) performed hands‑on reproducibility tests and aggregated model lists; their reporting is the primary driver that elevated the problem from forum anecdotes to vendor-level engagement.

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Technical hypotheses — what might be happening​

Several plausible, engineering‑sensible theories explain how an OS update could expose a drive‑level failure mode. These are hypotheses based on the symptom set; they are not confirmed root causes.

Host Memory Buffer (HMB) and OS buffer interaction​

Windows 11 24H2 previously changed HMB allocation behavior and host-side buffer handling, and earlier 24H2 rollouts exposed HMB-related instability on some WD/SanDisk drives — a precedent that makes host/firmware interaction a credible suspect. Under heavy sequential writes, HMB and controller metadata operations are stressed; if the OS changes timing, buffer allocation, or DMA behavior, that can trigger controller firmware assumptions and produce a hang or crash.

SLC caching exhaustion / DRAM‑less behavior​

DRAM‑less SSDs rely on SLC-mode caching and tighter coordination with the host for mapping updates. When the cache is exhausted — especially on drives that are already heavily used — sequential writes stress wear-leveling and metadata updates. If a firmware bug mishandles cache exhaustion or a host change alters the expected command timing, the controller can become unresponsive. Early reproduction data and the over‑representation of DRAM‑less/Phison entries make this a credible mechanism.

Metadata/firmware race or NVMe command queue edge-case​

The symptom of unreadable SMART and controller telemetry after the event suggests a controller-level hang or firmware crash rather than a pure file-system bug. If firmware state becomes corrupted during heavy writes, the drive may present as offline and require vendor reinitialization — which squares with some community reports of persistent inaccessibility.

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What is confirmed vs. uncertain​

Confirmed facts:

• Microsoft released KB5063878 for Windows 11 24H2 on August 12, 2025 (OS Build 26100.4946).
• Multiple community testers and specialist outlets have reproduced a consistent failure pattern where a drive disappears during sustained, heavy sequential writes and in some cases returns corrupted or inaccessible data. (tomshardware.com, borncity.com)
• Phison has publicly acknowledged and is investigating potential effects and is coordinating with partners.

Uncertain / still under investigation:

• Whether KB5063878 is the direct causal trigger, a coincidental timing correlation, or a catalyst that reveals latent firmware faults in certain controllers. Microsoft’s internal telemetry, as reported, had not shown an increase in failures at the time of company statements, which suggests the incident might be workload- and environment-specific.
• The precise firmware/controller families and combinations that produce the fault under real-world conditions remain to be exhaustively validated by vendors. Early model lists vary between testers.

Where reporting diverged or was later corrected:

• A falsified internal Phison document circulated in the community; Phison disavowed that document and pursued legal action. This highlights how fast misinformation can propagate in high‑visibility incidents — and why vendor statements should be treated as authoritative.

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Practical guidance — what users and IT teams should do now​

The evidence supports a cautious, data-first posture. The rules below are conservative because the failure can produce actual data corruption.

Immediate steps for consumers and power users​

• Back up critical data now. Use the 3‑2‑1 rule where practical: three copies, on two different media types, with one copy offsite. Do not rely on a single backup method.
• Avoid running sustained, large sequential writes (bulk game installs/updates, large archive extraction, cloning or mass-media copies) on Windows 11 24H2 systems until vendors and Microsoft publish mitigations or firmware updates. Community reproductions commonly used workloads of ~50 GB or more.
• Check your SSD vendor’s support site and tooling for firmware updates and advisories. Apply vendor‑recommended firmware only after verifying backups are in place. Firmware updates can both fix controller bugs and change recovery behavior — never update firmware without having a trusted backup.
• If you experience a drive disappearance, do not immediately reformat. Image the device with a forensic imaging tool (for example, ddrescue or vendor imaging tools) and contact vendor support — some vendors can recover metadata or apply controller-level reflashes. Capture Windows Event logs and collect vendor tool telemetry for support staff.

Recommendations for system administrators and fleet managers​

• Stage KB5063878 in a controlled pilot ring that includes representative storage hardware and heavy-write workloads. Do not push the update to broad production rings until vendor guidance and validated fixes arrive.
• Inventory storage hardware: map model, firmware version, and controller family so you can rapidly identify endpoints that match community‑identified leads. Prioritize backups for critical endpoints.
• Monitor Microsoft Release Health and vendor advisories for KIR (Known Issue Rollback), OOB (out‑of‑band) updates, or explicit blocking guidance that Microsoft can apply centrally. Microsoft has used servicing controls to mitigate other August regressions in this rollout window. (support.microsoft.com, tomshardware.com)

If you believe your drive is affected​

• Document the exact workload that reproduced the failure (size of transfer, source/destination drives, available capacity, steps to reproduce). This data is vital to vendor and Microsoft engineers.
• Use vendor diagnostic tools to capture SMART and controller logs if the drive becomes responsive; if not, preserve the device as-is and engage vendor support. Imaging first preserves recovery options.

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Risks of common “quick fixes” and myths to avoid​

• Uninstalling updates or rolling back a cumulative patch may temporarily remove the trigger on a system, but it is not a substitute for firmware-level remediation. If firmware is corrupted or the drive has suffered metadata loss, simply rolling back Windows will not restore on-disk integrity. Anecdotal user reports note isolated instances where uninstalling a related preview update appeared to help, but these are not general-purpose remedies and should be treated with caution.
• Attempting aggressive recovery actions (repartitioning, reformatting) without imaging first can destroy forensic evidence that would enable vendor recovery — and can convert a recoverable failure into permanent data loss. Always image before destructive steps.
• Trust vendor statements and coordinated advisories over unverified lists circulated on social media. As seen in this incident, counterfeit or falsified documents can spread quickly and amplify confusion.

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Deeper analysis: what this incident reveals about modern storage ecosystems​

This incident is an instructive case study in the fragility that can arise at the intersection of operating-system changes, driver behavior, and controller firmware logic.

• Modern SSD reliability depends on co‑engineering across multiple layers: OS storage stacks, device drivers, NVMe command handling, controller firmware, NAND management (SLC/LBA mapping, garbage-collection), and even motherboard/UEFI behavior. Small shifts in timing, buffer allocation, or command ordering on the host can expose latent firmware assumptions. The 24H2 branch has earlier surfaced precisely this class of issues.
• Community test benches and enthusiast reporting play an outsized role in early detection. Hobbyist reproducibility — when performed methodically — provides the practical signals vendors need to reproduce edge-case failures. That said, community lists can be noisy; robust telemetry and vendor validation remain essential for authoritative fixes.
• The governance of update delivery matters. Microsoft’s patching pipeline (multiple KBs, SSU/LCU combos, and staged rollout channels) can make it hard to predict how a single update will behave across diverse enterprise distribution systems (WSUS/SCCM) and consumer devices. The same update that is benign on one system may expose subtle bugs on another because of firmware variants, preinstalled drivers, or usage patterns. This incident underscores the ongoing operational tension between rapid security servicing and the need for representative hardware testing in production-like scenarios.

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Likely timeline and what to watch next​

• Short term: Microsoft and SSD vendors will continue to collect telemetry, attempt internal reproductions, and publish targeted advisories or firmware updates. Microsoft has requested customer feedback on affected systems and is working with partners to reproduce the issue.
• Mid term: expect vendor firmware updates for implicated controllers if a firmware bug is found, and/or an OS-level mitigation from Microsoft (an OOB patch or a Known Issue Rollback) if the problem can be addressed without firmware changes. Enterprise servicing controls may be used to block or roll back problematic packages from managed catalogs.
• Watch indicators: vendor support pages for firmware advisories, the Microsoft Release Health dashboard, and vendor diagnostics that explicitly reference KB5063878/KB5062660. Be cautious of unofficial model lists until vendors confirm affected SKUs and firmware revisions.

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Conclusion — a pragmatic posture for Windows users and admins​

The confluence of community reproducibility, vendor acknowledgement and Microsoft’s engagement elevates this beyond rumor to an operationally significant investigation. While evidence to date points to a workload-sensitive interaction — typical triggers: sustained sequential writes near ~50 GB on partly filled drives — the precise causal attribution remains under vendor and Microsoft forensic review. The responsible course for both consumers and administrators is conservative:

• Prioritize backups now.
• Avoid heavy sequential writes on systems that installed the August 12, 2025 KB until mitigations are published.
• Stage the update in representative pilot rings and inventory storage hardware precisely.
• Follow vendor firmware advisories and Microsoft Release Health guidance, and preserve logs and images if you encounter an affected device. (tomshardware.com, bleepingcomputer.com)

This is an unfolding technical story where community ingenuity and vendor collaboration will determine how quickly and cleanly it is resolved. The immediate imperative for all Windows users remains simple and immutable: back up valuable data and avoid risk‑heavy storage operations on patched systems until the engineers close the loop.

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Source: BornCity Windows 11 24H2: Microsoft investigates reports of SSD issues caused by KB5063878 | Born's Tech and Windows World