# Why Microsoft Compatibility Telemetry Uses High Disk on Windows Microsoft Compatibility Telemetry in Task Manager is `CompatTelRunner.exe`, a Windows program in `C:\Windows\System32` that a scheduled task starts to check this PC's apps and drivers against newer versions of Windows. While it runs, it reads a lot of files, so on a hard drive or a busy SSD it can push disk use to 100% for several minutes. Most of the time the right move is to let one run finish, then decide whether its schedule or your diagnostic data setting needs changing. ## What starts it Microsoft's [compatibility scan article](https://learn.microsoft.com/en-us/troubleshoot/windows-client/setup-upgrade-and-drivers/use-windows-setup-compatibility-scan-logs-to-identify-blocking-issues) names the component: Microsoft Compatibility Appraiser runs as the `\Microsoft\Windows\Application Experience\Microsoft Compatibility Appraiser` scheduled task and writes what it finds under `HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AppCompatFlags`. The same article says the appraiser processes write and refresh compatibility information whenever they run a scan. Windows uses that information to judge whether a newer version would run into trouble with something installed on the PC. So the disk activity is reading, not filling the drive. A run that started while you were away can still be going when you come back, and after a large app install or a Windows update there is more to look at. ## Confirm it is the busy process Open Task Manager with `Ctrl + Shift + Esc`, sort by Disk and look for Microsoft Compatibility Telemetry. On the Details tab, add the Command line column (right-click a column heading, Select columns) and check that the program is `C:\Windows\System32\CompatTelRunner.exe`. A file with the same name anywhere else is not the Windows component; scan it with Windows Security. To see what it is reading, open Resource Monitor (type `resmon` in Start), go to the Disk tab and tick `CompatTelRunner.exe`. The Disk Activity list shows the files and the read rate. Program Files folders, driver stores and app packages are the expected targets. To see how often it runs, open Task Scheduler, go to Task Scheduler Library › Microsoft › Windows › Application Experience and select Microsoft Compatibility Appraiser. The Last Run Time and Next Run Time columns, and the Triggers tab, show the schedule on this PC. ## Let one run finish Leave the PC on and give it a few minutes. When the run ends, the process exits and the disk graph drops back. If it happens at a bad time every day, that is a scheduling question, not a fault. If disk use stays at 100% after Compatibility Telemetry exits, the cause is something else, and [Disk at 100% in Task Manager](/windows/blog/windows-disk-100-percent) covers the rest. A hard drive is the usual reason this scan feels heavy: the same reads that pass unnoticed on an SSD keep a spinning disk busy. ## Choose how much diagnostic data Windows sends Windows 11 has two user-facing diagnostic data levels, Required and Optional; Microsoft's [diagnostic data reference](https://learn.microsoft.com/en-us/windows/privacy/configure-windows-diagnostic-data-in-your-organization) lists Required (formerly Basic) and Optional (formerly Full), and notes that the old Enhanced level does not exist on Windows 11. To send only the required set, open Settings › Privacy & security › Diagnostics & feedback and turn off Send optional diagnostic data. On a work or school PC this can be set by policy, and the page then says some settings are managed by your organization. This setting changes what is sent to Microsoft. Turning it off alone does not stop the appraisal task from running on schedule. ## Disabling the task works, at a cost On a PC you manage yourself, you can right-click Microsoft Compatibility Appraiser in Task Scheduler and choose Disable. It is reversible with Enable. The cost is that Windows no longer refreshes its compatibility record for this PC, which is the information it relies on before offering a feature update. Some users on Microsoft's forums report the task coming back enabled after an update, so check it again after a feature update rather than assuming it stayed off. Changing the trigger costs less than disabling the task. The Triggers tab shows when it fires, and on a PC left on overnight the scan runs while nobody is using it. If you change the task, note what you changed so you can put it back. ## What not to do - Do not delete or rename `CompatTelRunner.exe`. It is a Windows component owned by TrustedInstaller, removing it means taking ownership of a system file, and a later update or repair can restore it. - Do not run "debloat" scripts that disable dozens of services and tasks at once to stop one scan. You lose track of what changed, and some of it breaks Windows Update. - Do not end the process repeatedly in Task Manager. The next scheduled run starts it again, and an interrupted run has to do the work over. ## Where Mole fits Mole for Windows does not change diagnostic data settings or Windows' own scheduled tasks; its startup list covers tasks outside `\Microsoft\Windows`, so the Compatibility Appraiser never appears there to switch off. On the Status page, the process list shows Microsoft Compatibility Telemetry with its CPU and memory while it runs. Choosing Why is this running? from the row's menu shows the program file and which process started it, and Show in File Explorer opens its folder for the location check above. Because it runs from the Windows folder, Mole does not offer Terminate for it. For other names in Task Manager, see [Windows processes explained](/windows/blog/windows-processes-explained). ## FAQ ### Is Microsoft Compatibility Telemetry a virus? The genuine program is `CompatTelRunner.exe` in `C:\Windows\System32`, started by the Microsoft Compatibility Appraiser scheduled task. A file with that name elsewhere should be scanned. ### Why does Microsoft Compatibility Telemetry use so much disk? It reads installed programs and drivers to refresh Windows' compatibility record. On a hard drive those reads can hold the disk at 100% for several minutes until the run ends. ### Can I disable Microsoft Compatibility Telemetry? You can disable the Microsoft Compatibility Appraiser task in Task Scheduler on your own PC, and enable it again later. Windows then stops refreshing compatibility data it uses before feature updates. ### Does turning off optional diagnostic data stop it? It limits what Windows sends to the required set. It does not, by itself, stop the scheduled appraisal from running. Canonical: https://mole.fit/windows/blog/microsoft-compatibility-telemetry-high-disk-windows