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If Claude Cowork or WSL stopped showing Windows folders after an update, first identify which access path failed. Microsoft documented a September 2026 Windows security-update regression that could break Plan9 folder sharing for both applications while the Linux VM still started. Microsoft says Windows updates released on or after September 14, 2026, including KB5129195, resolve that issue. Install the update applicable to your Windows build and restart; if the problem remains, check WSL’s separate drive-mount settings or investigate Cowork’s remaining mount failure.
Identify which shared-folder path is broken
Cowork’s sandbox shell, Cowork’s file tools, and a regular WSL distribution do not use identical paths to access Windows folders. A missing folder in one does not necessarily mean the folder is missing from Windows or inaccessible everywhere.
| What you see | Likely path to investigate |
|---|---|
| Cowork’s shell reports that no Plan9 drive shares were mounted, or cannot see a connected folder | Cowork’s VM-to-host Plan9 sharing. Microsoft’s update notice specifically describes this class of failure. |
| Cowork’s shell cannot mount a folder, but Cowork’s file tools can list, stage, or commit files | The shell mount path may be failing independently of Cowork’s file-tool access. A Cowork issue report documents this combination; it does not establish that the folder has disappeared from disk. |
A regular WSL distribution has no /mnt/c |
WSL’s DrvFs automount configuration, which is separate from Cowork’s Plan9 shares. |
| Cowork itself will not start | Check the Windows virtualization feature and services, rather than treating this as only a missing-folder problem. |
Anthropic describes Cowork as running code in an isolated virtual machine, with reads and writes limited to folders the user connects. That means a shell mount failure and working file tools can reflect different access paths.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsInstall the Windows update that addresses the September 2026 regression
Microsoft documented that after the September 2026 security update KB5124008, applications using HCS-managed Linux virtual machines could have trouble sharing host folders with Linux VMs through Plan9. Microsoft specifically names Claude Cowork and WSL as affected applications. Affected VMs may start normally even when shared folders are missing or inaccessible. See Microsoft’s Windows 11 KB5124008 support page.
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- Open Settings > Windows Update and check for updates.
- Install the applicable Windows update published on or after September 14, 2026. Microsoft gives KB5129195 as an example of an update that resolves the documented issue.
- Restart Windows, then test the same folder in the access path that failed.
Update names and applicability depend on your Windows version and servicing branch. Microsoft’s cited notice covers Windows 11 24H2 and 25H2 update builds 26100.9445 and 26200.9445 for KB5124008. Use Windows Update or a package intended for your own branch; do not force a standalone update that does not apply to your build. A Windows Insider Beta report says KB5129195 did not apply to build 26220.9472 because that build belonged to a different, higher branch.
One Cowork issue reporter said uninstalling KB5124008 and rebooting restored their shares. That is an individual report, not Microsoft’s recommended general remedy; prefer the applicable corrective update over removing a security update.
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Restore a missing /mnt/c in ordinary WSL
Microsoft documents that fixed Windows drives are mounted in WSL through DrvFs under /mnt by default, so the C: drive normally appears at /mnt/c/. WSL configuration can disable automount or change its mount root. If only a regular WSL distribution is affected, check its automount settings rather than assuming Cowork’s Plan9 issue explains the symptom. See Microsoft’s WSL automount settings documentation.
- Confirm that fixed-drive automount is enabled in the distribution’s WSL configuration.
- Check that the configured automount root is the expected
/mnt/. If the root was changed, the C: drive may be mounted somewhere other than/mnt/c/. - After correcting a setting, restart the WSL distribution and check the configured mount location again.
If WSL’s mount configuration is correct and drives are still absent after the applicable Windows update and restart, record the Windows build and update alongside the exact WSL symptom. That distinguishes a configuration issue from a continuing Windows-level failure.
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Check virtualization setup if Cowork will not start
If the problem is that Cowork itself cannot start—not merely that its shell cannot see a connected folder—Anthropic’s Windows troubleshooting guidance says to check whether Virtual Machine Platform is enabled and whether the vmcompute and hns services exist. If the feature is disabled or the services are missing, Anthropic’s guidance is to re-enable the feature and restart Windows. See Anthropic’s Claude Desktop troubleshooting guide.
These checks address missing virtualization setup; they are not a verified fix for every Plan9 sharing regression. If Cowork starts but its shell still cannot mount a connected folder, use the Windows update path and then report the continuing mount failure rather than assuming the virtualization checks will resolve it.
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If Cowork still cannot mount folders after updating
Microsoft’s resolution does not establish that every Cowork setup will recover. A separate Cowork report describes shell mount failures after KB5129195 even though Cowork’s file tools could list, stage, and commit files. The report is an individual account, not evidence of how common the issue is. The available information does not establish a Claude Desktop release that fixes all post-update mount failures.
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When contacting support or filing a bug report, include the details needed to distinguish a branch mismatch, a shell-only failure, and a broader Cowork problem:
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- Your exact Windows version and OS build.
- The installed Windows update identifier, such as KB5124008 or KB5129195, and whether Windows Update offers a newer applicable update.
- Your Claude Desktop version.
- The exact error text and whether the VM starts.
- Whether Cowork’s file tools can access the connected folder even when the sandbox shell cannot.
Individual reports describing these symptoms include a Cowork report following KB5124008 and a report of continued shell mount failures after KB5129195. Treat them as symptom reports, not confirmed general fixes.
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