Troubleshooting Windows 10 Large File Copy Failure to QNAP NAS

A few days ago, I purchased a QNAP TS-453B mini. After setting up the Samba share, I happily attempted to copy files from Windows 10 to the NAS. Small files worked fine, but when copying a 6GB file, the speed was initially good, then dropped to near zero, and finally the transfer interrupted without any error messages. Repeated attempts with large files yielded the same result.

Checking the Router

First, I disabled QoS speed limiting on the router and tried copying the large file again, but the result was the same. It did not seem to be a router issue.

Checking Windows Firewall

Next, I disabled the built-in Windows firewall and completely turned off the antivirus software "Huorong," but the transfer still failed.

Switching Protocols and Using FTP

Windows copies files to NAS using the Samba protocol. Suspecting a Windows 10 bug, I decided to switch protocols and use FTP software to transfer the large file, but it failed again.

Final Solution

After trying various exclusion methods without finding the cause, I was at a loss. However, I accidentally noticed that the available space on the C drive decreased as I copied the large file, eventually dropping to 0, at which point the copy speed also became 0 before failing. I suspected this was related to C drive space redundancy.

The file being copied was 6GB, but the available space on the C drive was less than 4GB. I began cleaning up the C drive space, freeing up about 8GB of free space. I then tried copying the large file again; this time, the speed was stable, and the failure did not occur. Therefore, I suspect that Windows 10 occupies temporary space on the C drive when copying files to a NAS. If the temporary space is insufficient, the transfer cannot continue and fails.

Conclusion

Although the problem was eventually resolved, it was quite frustrating. Windows 10 showed no errors, and there were no error messages in the Event Viewer. It was only by accidentally noticing that the available space on the C drive dropped to 0 during the copy process that I found the clue. If you encounter similar issues, it is worth checking if the available capacity on the C drive is sufficient. It is recommended to reserve ample space on the C drive to avoid various strange problems.