OneDrive Admin Checklist: file server migration misses files for mergers

When migrating file server data to OneDrive for a merger or acquisition, you may discover that some files are missing after the transfer. This problem typically occurs because of file path length limits, unsupported characters, or permission inheritance conflicts that the migration tool did not report. This article explains the root causes of missing files during a merger migration, provides a detailed checklist to verify completeness, and describes how to locate and manually recover skipped items.

Why File Server Migrations Miss Files During Mergers

File server migrations to OneDrive fail to copy some files for three primary reasons. First, Windows file paths that exceed 400 characters are blocked by OneDrive and SharePoint sync limits. Second, file names that contain characters such as ~, #, %, or leading/trailing spaces are rejected by the SharePoint storage engine. Third, files that inherit unique permissions from a parent folder that was not migrated are silently skipped by most migration tools.

During a merger, the source file server often contains decades of nested folder structures, legacy naming conventions, and custom permission sets. The default migration settings in the SharePoint Migration Tool or third-party tools may not flag every skipped file. The result is a OneDrive site that appears complete but is missing critical business documents.

A secondary cause is the use of the OneDrive sync app as a migration method. Dragging folders from a file server into a synced OneDrive folder does not preserve metadata, timestamps, or permissions. The sync app also stops copying when it encounters a single file that violates sync rules, leaving the remainder of the folder untransferred.

Pre-Migration Checklist to Prevent Missing Files

Before starting the migration, run the following checks on the source file server. Each step reduces the chance of skipped files.

1. Scan for file path length violations
   Open a PowerShell session as administrator and run Get-ChildItem -Path "\\Server\Share" -Recurse | Where-Object { $_.FullName.Length -gt 400 } | Export-Csv -Path "C:\longpaths.csv". Review the CSV and rename or move any files that exceed 400 characters. OneDrive and SharePoint have a 400-character path limit including the site URL.
2. Identify invalid file name characters
   Use the SharePoint Migration Tool pre-scan feature. In SPMT, select your source, click Scan, and review the report under Issues. Files with ~, #, %, &, {}, [], or trailing periods will appear as warnings. Rename these files on the source before migration.
3. Audit folder permissions
   Run icacls "\\Server\Share" /t /save "C:\perms.txt" to export current NTFS permissions. Compare this list with the permissions that SPMT will map. If a file inherits permissions from a folder that will not be migrated, break the inheritance and apply explicit permissions to the file.
4. Count total files and folders
   Run Get-ChildItem -Path "\\Server\Share" -Recurse -File | Measure-Object | Select-Object Count to get the file count. Record this number. You will compare it against the destination count after migration.

Post-Migration Verification Steps

After the migration tool finishes, do not assume all files are present. Complete these verification steps on the destination OneDrive site.

1. Compare file counts in SharePoint Admin Center
   Go to Microsoft 365 admin center > Reports > Usage > OneDrive files. Locate the destination site and note the total file count. Compare it with the source count you recorded earlier. A discrepancy of more than 1% indicates skipped files.
2. Run a delta scan with SPMT
   In the SharePoint Migration Tool, create a new migration job with the same source path. Select Scan only mode. SPMT will compare the source against the destination and list any files that are missing or have different modified dates.
3. Check the migration report JSON
   SPMT generates a JSON report at C:\Users\[Admin]\AppData\Local\Microsoft\SPMT\Reports\[JobId]\ScanResult.json. Open this file in a text editor and search for "Status":"Skipped". Each skipped entry includes a "SkippedReason" field that explains why the file was not copied.

If OneDrive Still Has Issues After the Main Fix

Files are present but show a gray sync icon

A gray icon means the file is online-only and not synced to the local device. This is not a missing file. Instruct users to right-click the file and select Always keep on this device if they need offline access. The file is stored in OneDrive and accessible via the web.

Files are listed in the web interface but not in File Explorer

This occurs when the sync app has not downloaded the folder structure yet. Open OneDrive settings, select Sync and backup > Manage backup, and confirm that the folder is included in the sync scope. If the folder is excluded, add it and wait for the sync to complete.

Migration tool reports success but files are still missing

Some third-party tools mark a migration as successful when the transfer completes at the root level, even if subfolders were skipped. Use the SPMT delta scan method described above to find the exact files. Once identified, copy them manually using the OneDrive web upload feature or a fresh SPMT job with the Replace option set to Skip to avoid overwriting existing files.

File Server vs OneDrive for Business: Key Differences for Mergers

Item	File Server	OneDrive for Business
Maximum file path length	32,767 characters (Windows 10/11)	400 characters (including site URL)
Invalid characters in file names	Only \ / : ? " < > | are blocked	Blocks ~ # % & { } [ ] and leading/trailing spaces
Permission inheritance	NTFS inheritance from parent folders	SharePoint unique permissions; inheritance must be explicit
File size limit	Determined by disk capacity	250 GB per file
Version history	Shadow Copies (VSS) or third-party backup	500 versions per file automatically
Migration tool support	N/A	SPMT, Mover.io, ShareGate, Metalogix

During a merger, the path length and character restrictions in OneDrive cause the majority of skipped files. Plan for at least two weeks of remediation time to rename or relocate files that violate these limits.

After completing the migration checklist and verifying file counts, you can be confident that the destination OneDrive site contains all required business data. For future mergers, run the SPMT pre-scan at the start of the project and schedule a delta scan one week after the initial migration to catch any files that were modified during the cutover window. As an advanced tip, use the Set-SPOTenantSyncClientRestriction PowerShell cmdlet to block sync of file types that are known to cause migration failures, such as .tmp and .bak files, before the migration begins.