1
Diagnostic step 1
Capture a packet trace on client and NAS during a reproducer (Wireshark/tcpdump) focusing on SMB2/SMB3 WRITE/FLUSH and associated ACKs. Keep captures short to protect privacy; do not share file contents.
FULL AI ANALYSIS
Windows 11 SMB file copy stalls at 100% for multi-gigabyte transfers to NAS
Generated Oct 3, 2026, 12:35:03 AM
Analysis summary
Symptoms indicate SMB writes progress on the client but the server side stops accepting or acknowledging final writes/flushes. Possible areas: client-side caching/buffering causing apparent completion before server flush, SMB protocol mismatch or opportunistic locking issues, NIC offload or checksum offload interacting with SMB large writes, NAS SMB implementation bug handling high-offset WRITE/FLUSH, or intermediate network device (switch/firmware) dropping specific packet sequences. The fact forcing SMBv2 avoids the hang suggests a protocol/implementation interaction.
Possible causes
SMB protocol feature mismatch: SMB3 features (multichannel, encryption, signing) interacting poorly with NAS firmware causing write/flush deadlock.
85%NAS SMB server bug or resource exhaustion handling high-offset writes (particularly with RAID reconstruction or dedupe enabled).
80%SMB client aggressive write caching or local buffering reports completion before server flush; flush/close not acknowledged and retry logic stalls.
55%Network device (switch/coaxial) or NIC offload (large send offload/TSO/GRO) corrupts or drops certain packets leading to stalled SMB session.
20%Suggested fixes
2
Diagnostic step 2
On a test client, disable NIC offloads (Large Send Offload, TCP checksum offload) temporarily: use PowerShell Set-NetAdapterAdvancedProperty or NIC vendor tool, then retry copy to see if behavior changes.
3
Diagnostic step 3
Temporarily disable SMB caching on client by setting DisableFileSync and related registry keys for SMB to force synchronous writes; test small reproducer files first.
4
Diagnostic step 4
Try mounting the share with explicit SMB version (mount -o vers=2.0 equivalent) or disable SMB 3 features on NAS to isolate protocol differences, then test. Revert settings after test.
5
Diagnostic step 5
Check NAS logs for SMB errors, memory pressure, or deduplication/antivirus on-access processes during transfer; run disk/scrub health checks in maintenance window before destructive operations and avoid RAID rebuilds during tests. If packet captures show stalled FLUSH/ACKs and disabling offloads fixes it, update NIC firmware/drivers and switch firmware. If NAS shows errors or reproduces on another NAS with same firmware, escalate to NAS vendor with captures and logs. If uncertain, stop large transfers to avoid corruption and consult storage admin.