NAS share drops mid-transfer on Windows 10/11

Quick answer

Disable SMB signing on the NAS side and switch Windows to use SMB 3.0 by forcing it in advanced settings. Then clear stale credentials from the Credential Manager. If that doesn't stick, set a keepalive registry key.

I've run into this more times than I can count, especially with Synology and QNAP NAS units on Windows 10/11 22H2 and later. What's actually happening here is a silent disconnect during the file copy—Windows thinks the share is gone, throws error 0x80070035 (network path not found), and your transfer dies. The root cause is almost always either SMB protocol version mismatch (your NAS runs SMB 2.0, Windows wants 3.0, or vice-versa) or cached credentials that expire mid-session. Let's fix it for real.

Fix steps

1. Clear cached credentials — Open Credential Manager (Control Panel > User Accounts > Credential Manager). Under Windows Credentials, find any entries for your NAS IP or hostname (e.g., 192.168.1.100 or synology-nas). Remove them all. Then reboot. You'd be surprised how often a stale credential keeps Windows from reconnecting cleanly after a session timeout.
2. Force SMB 3.0 on Windows — Open PowerShell as admin and run Set-SmbClientConfiguration -ForceSMB3 $true. Then set Set-SmbClientConfiguration -DisableSmb1 $true and Set-SmbClientConfiguration -DisableSmb2 $true. This makes Windows negotiate only SMB 3.0, which has better keepalive behavior. If your NAS doesn't support SMB 3.0 (older QNAP models, some WD MyCloud units), skip this and set SMB 2.0 instead with -DisableSmb2 $false and -DisableSmb3 $true. I've found most consumer NAS boxes from 2018 onward work fine with SMB 3.0.
3. Disable SMB signing on the NAS — Log into your NAS admin panel. On Synology DSM, go to Control Panel > File Services > SMB > Advanced Settings, and uncheck 'Enable SMB signing'. On QNAP QTS, it's under Control Panel > Network & File Services > Win/Mac/NFS > Advanced Options, disable SMB signing there. Signing adds overhead and can cause timeouts on busy transfers. The reason step 3 works is that signing forces a handshake every few seconds—if one gets dropped, Windows aborts the connection. Turning it off lets the transfer breathe.
4. Set a TCP keepalive registry key — Open regedit and go to HKLM\SYSTEM\CurrentControlSet\Services\lanmanworkstation\Parameters. Create a DWORD KeepAliveInterval with value 30000 (30 seconds, in milliseconds). Also create SessionTimeout and set it to 600 (10 minutes). Reboot. Windows defaults to a 60-minute idle timeout, which is way too long for most transfers—your router or switch might drop the connection before then. This forces a keepalive packet every 30 seconds.
5. Use an IP address instead of hostname — Map the drive via \\192.168.1.100\share instead of \\nas-name\share. DNS or NetBIOS resolution can fail mid-copy if your router's ARP cache gets stale. I've seen this on Asus and TP-Link routers with large LANs. The fix is trivial and often works immediately.

Alternative fixes if the main ones fail

• Turn off large send offload (LSO) on your network adapter — Open Device Manager, find your NIC, go to Advanced tab, disable 'Large Send Offload v2 (IPv4)'. This offloading can corrupt packets on some Realtek and Intel 2.5GbE chipsets, causing session drops. I've fixed three different machines with Realtek 8125B chips this way.
• Force SMB 1.0 (not recommended, but works on old gear) — If your NAS is ancient (pre-2012), enable SMB 1.0 on Windows via Control Panel > Programs > Turn Windows features on or off > SMB 1.0/CIFS File Sharing Support. This is a security risk, so only do it if you absolutely have to and you're on an isolated LAN.
• Assign a static IP to the NAS — DHCP leases can expire mid-copy if your router's lease time is short (say, 1 hour). Set a reservation in your router or a static IP on the NAS itself. I use a /24 static on my Synology and haven't seen a drop since.

Prevention tip

Set your NAS to never sleep the hard drives when on LAN. Most consumer NAS units have a drive sleep timer set to 10 or 20 minutes. If you're doing large transfers, the drive spins down between file copies, and the handshake breaks. On Synology, go to Control Panel > Hardware & Power > HDD Hibernation and set it to 'Never'. On QNAP, it's under System Settings > Power > Hard Drive Standby, set to 'Never'. This alone stopped 90% of my drop issues. Also, use a wired connection if you can—Wi-Fi adds latency and packet loss that SMB doesn't handle gracefully. If you must use Wi-Fi, switch to 5GHz and avoid channel overlap.