A few days ago, I bought a QNAP TS-453B mini and set up Samba sharing. I was happily copying files from Windows 10 to the NAS. Small files were no problem, but when I tried to copy a 6GB file, the speed started off well and then gradually dropped to zero, eventually failing without any error messages. I tried again with other large files, but encountered the same issue.

Checking the router

First, I disabled QoS speed limiting on the router and tried copying the large file again. However, the issue persisted, so it didn't seem to be a router problem.

Checking Windows Firewall

Next, I disabled the built-in Windows Firewall and completely shut down the antivirus software "Huorong", but the issue still persisted.

Trying a different protocol - FTP

Since the file copying from Windows to the NAS was using the Samba protocol, I thought it might be a bug in Windows 10. So I decided to try a different protocol and used FTP software to transfer the large file, but it still failed.

Final solution

After trying various troubleshooting steps, I couldn't find the root cause of the issue. However, I noticed that when copying large files, the available space on the C drive kept decreasing until it reached zero, at which point the copying speed also dropped to zero and failed. This made me suspect that the issue might be related to insufficient temporary space on the C drive.

The file I was copying was 6GB in size, but the available space on the C drive was less than 4GB. So I started cleaning up the C drive and freed up around 8GB of space. After that, I tried copying the large file again and this time the speed was stable without any failures. Therefore, I suspect that when copying files from Windows 10 to the NAS, Windows 10 uses temporary space on the C drive for buffering, and if the temporary space is not sufficient, the transfer fails.

Conclusion

Although I eventually solved the problem, it was quite frustrating. Windows 10 didn't show any errors, and there were no error messages in the Event Viewer. I only discovered that the available space on the C drive was decreasing to zero during the file copying process. If anyone else encounters a similar issue, I suggest checking the available capacity of the C drive. It's recommended to reserve sufficient space on the C drive to avoid various strange problems.