SharePoint Migration Tool deep dive

The SharePoint Migration Tool (SPMT) and Migration Manager are Microsoft's free first-party tools for migrating content from on-premises SharePoint, file shares, and other clouds into SharePoint Online and OneDrive. For larger migrations, knowing how to drive them well — and where their limits are — matters.

SPMT vs Migration Manager

Two related tools:

• SharePoint Migration Tool (SPMT) — a Windows desktop client you run on a machine with access to the source content. Best for one-off migrations or jobs from a single source.
• Migration Manager — a web-based orchestrator in the SharePoint admin center that drives multiple SPMT agents across distributed source environments. Best for large-scale, multi-source projects.

Same underlying engine; Migration Manager adds the orchestration and reporting layer.

What can migrate

• Windows file shares — UNC paths, mapped drives, on-prem shared folders.
• On-premises SharePoint — SharePoint 2010 / 2013 / 2016 / 2019 / Subscription Edition site collections.
• Google Drive — including Shared Drives via the Migration Manager Google integration.
• Box — via Migration Manager.
• Dropbox — via Migration Manager.

Each source has its own pre-migration considerations.

Preparation

The success of a migration depends mostly on preparation, not on running the tool. Pre-migration tasks:

Inventory and triage

Walk the source and identify:

• Total volume (GB and file count).
• Owners for each share / site.
• Stale content that shouldn't migrate.
• Files with invalid characters or paths too long for SharePoint.
• Permissions complexity — NTFS ACLs map imperfectly to SharePoint permissions.

A common finding: 30–60% of source content hasn't been accessed in years. Migrate selectively.

Target design

Design where each source goes:

• Personal H: drives → user OneDrives.
• Department shares → department SharePoint team sites.
• Project shares → Teams teams.
• Reference content → communication sites.

Resist replicating folder structure 1:1. Flatten and use metadata.

Pre-create destinations

Create the target sites, libraries, and permissions before migration. The tool moves content into existing locations more cleanly than provisioning on the fly.

Running the migration

A typical wave:

1. Run a scan first to identify migration issues without moving content.
2. Review the scan report — fix file names, shorten paths, remove unsupported content.
3. Run a pilot migration of one share to validate process and timing.
4. Schedule the bulk migration during a maintenance window or off-hours.
5. Run an incremental delta after the bulk to catch any newly modified files.
6. Cutover — make the source read-only, run a final delta.
7. Validate — spot-check files post-migration.
8. Decommission the source after a stabilisation period (30 days typically).

Migration Manager features

For larger migrations, Migration Manager adds:

• Multi-agent orchestration — SPMT agents on multiple source machines for parallelism.
• Web-based reporting — task status, throughput, errors, success rate.
• Bandwidth control — limit migration impact on the source network.
• Scheduling — run migrations during specific windows.
• Migration assessments — built-in scanning for issues before migration.

Performance

Realistic throughput depends on many factors:

• Bandwidth between source and Microsoft cloud — usually the bottleneck.
• File sizes — many small files migrate slower per GB than few large files.
• Source filesystem performance — slow source storage slows the whole operation.
• Microsoft cloud throttling — Microsoft rate-limits ingestion; small bursts are fine, sustained heavy load gets throttled.

Plan for hundreds of GB to single-digit TB per day per source machine in typical scenarios.

Third-party alternatives

For migrations Microsoft's free tooling can't handle well — complex permission preservation, multi-system source heterogeneity, custom logic — third-party tools (ShareGate, AvePoint Fly, Quest Migration Manager, BitTitan) offer richer features at a price.

For most file-share-to-SharePoint and SharePoint-on-prem-to-SharePoint Online migrations, SPMT and Migration Manager are sufficient. For complex hybrid SharePoint scenarios or large M&A migrations, third-party tools usually win on ROI.

After migration

• User onboarding — show users where their content is now.
• Roll out OneDrive Known Folder Move if not already done.
• Decommission the old source — leaving it accessible delays the cultural transition.
• Capture lessons for the next wave.

A successful migration replaces "where are the files?" with "the files are in the obvious place." That's the goal — not just moving bytes from A to B.