During a weekend cutover from a legacy file server to OneDrive, administrators often discover that a subset of files did not migrate. This happens when users have files open, sync clients are left running, or network interruptions occur during the migration window. The result is a mismatch between the source file server and the destination OneDrive tenant. This article explains the root causes of missing files and provides a practical checklist to prevent and recover from these gaps.
Key Takeaways: File Server Migration to OneDrive Weekend Cutover
- Migration tool validation report: Run a delta scan after the cutover to identify files that were not copied due to locks or path length violations.
- OneDrive sync status check: Verify that Known Folder Move and sync relationships are active for all affected user accounts before the cutover.
- File server audit logs: Review Windows Event ID 4663 to confirm which files were accessed or changed during the migration window.
Why File Server Migrations Miss Files During Weekend Cutovers
Weekend cutovers impose a tight timeframe. Migration tools such as Microsoft SharePoint Migration Tool or third-party products like Quest or AvePoint copy files in batches. If a file is locked by a user or application, the tool skips it and logs a warning. Many administrators do not review these warnings until after the cutover, when users report missing files.
Another common cause is the use of file paths that exceed the 400-character limit in OneDrive and SharePoint. Files nested deep in the file server hierarchy are silently skipped. Additionally, if the OneDrive sync client is running on a user’s machine, it can hold file handles that prevent the migration tool from reading the file. The tool interprets the handle as a lock and moves on.
Network instability during the migration window also contributes. When a tool loses connectivity to the file server, it may mark the batch as complete even though some files were not transferred. The tool’s log shows a transfer error, but administrators often miss this in the noise of a large migration.
File Locking and Open Handles
Windows file servers use opportunistic locking. When a user opens a file, the server grants a lock that prevents other processes from reading or writing. Migration tools attempt to open files with read access. If the lock is exclusive, the tool retries a few times and then skips the file. The default retry count in many tools is three attempts with a 30-second interval. A user who leaves a file open overnight causes the tool to skip that file.
Path Length and Special Characters
OneDrive and SharePoint have a path length limit of 400 characters. File server paths longer than this are truncated or skipped. Special characters such as ampersands, question marks, and leading spaces also cause skips. The migration tool logs these as errors, but the log file is often not checked until after the cutover.
Checklist to Prevent Missing Files During Weekend Cutovers
- Run a pre-migration inventory scan
Use a tool like Microsoft SharePoint Migration Tool scan mode or TreeSize to list all files, their paths, and their sizes. Export the list to a CSV. This creates a baseline to compare against the post-migration inventory. - Notify users to close all files 24 hours before cutover
Send a mandatory email and a Teams broadcast. Instruct users to log off their machines. For shared network drives, ask IT to schedule a restart of the file server to release all file handles. - Disable the OneDrive sync client on user machines
Use Group Policy or Intune to stop the OneDrive sync client service before the migration window begins. A running sync client holds file handles on any folder it is syncing, including Known Folder Move locations. - Run a delta migration after the initial copy
Most migration tools support a delta pass that copies only files that changed or were missed. Schedule a delta pass immediately after the full copy. Set the delta pass to run with unlimited retries and a 10-second retry interval. - Review the migration tool error report
Open the report generated by the tool. Filter for errors with code 0x80070020, which indicates a file lock. Also filter for errors with code 0x8007007B, which indicates a path length violation. Re-run the migration for those specific files. - Compare file counts between source and destination
Use PowerShell to count files in the source folder and the destination OneDrive or SharePoint document library. Run this command on the source: Get-ChildItem -Recurse -File | Measure-Object. Run this command on the destination: Get-PnPFolderItem -FolderSiteRelativeUrl -ItemType File | Measure-Object. The counts must match. - Enable verbose logging on the migration tool
Set the logging level to Verbose before the cutover. This captures every file attempt and the reason for any skip. Use the log to identify patterns such as a specific folder where all files failed. - Test a dry-run cutover on a representative folder
Select a folder with more than 500 files and nested subfolders. Run the full migration process on this folder. Check the result. If files are missing, adjust the retry settings, path handling, or user notification process before the actual cutover.
If Files Are Still Missing After the Weekend Cutover
OneDrive shows fewer files than the file server
Run a delta migration again with the migration tool set to Overwrite mode. This mode replaces existing files in OneDrive with the source version. Use this only for files that were missed, not for all files. To target specific files, export the error report from the initial migration, extract the file paths, and create a CSV input file for the tool.
Users report missing files that were present before the cutover
Check the file server audit log for event ID 4663. This event records every access attempt. If the event shows a read access by a user account during the migration window, the file was likely locked. Ask the user if they had the file open. Then copy the file manually from a backup or from the user’s local machine.
Migration tool reports success but files are not in OneDrive
This can happen when files are uploaded to a different folder than expected. Check the migration tool mapping file. The mapping might have redirected files to a different document library or subfolder. Use the SharePoint admin center > Content explorer to locate the files by their content hash. If the files are in the wrong location, move them using SharePoint migration or PowerShell Move-PnPFile.
File Server Migration vs OneDrive Sync: Key Differences
| Item | File Server Migration Tool | OneDrive Sync Client |
|---|---|---|
| File handling | Copies files in bulk, skips locked files | Syncs files individually, retries on lock |
| Path length limit | 400 characters for OneDrive target | 400 characters for OneDrive target |
| Delta support | Yes, with manual scheduling | Automatic, continuous |
| Error reporting | CSV log with error codes | OneDrive activity center and sync status |
| User involvement | Low, IT runs the process | High, user must sign in and accept sync |
| Best use case | One-time bulk migration | Ongoing file synchronization |
Using a migration tool for the initial bulk copy and the OneDrive sync client for ongoing updates gives the best coverage. The migration tool handles the heavy lifting, and the sync client catches any files that were missed during the cutover.
After the cutover, you can run a final delta migration using the Microsoft SharePoint Migration Tool with the flag -SkipFileLockCheck. This flag forces the tool to attempt reading locked files. Use it only during a maintenance window when users are logged off. This step typically recovers the last 1 to 2 percent of missing files.
For future migrations, consider using the Known Folder Move policy to redirect Desktop, Documents, and Pictures to OneDrive before the cutover. This reduces the number of files that need to be migrated from the file server and gives users a familiar sync experience from day one.