This is an SMB issue. Windows Server Backup will fail in some conditions. Please refer to the table below.
If the error message is "The specified backup disk cannot be found", please try the following solution.
- Use a compatible third-party backup tool. For more information, please visit www.qnap.com.
- Create or attach a vhd(x) file as a local drive (diskmgmt.msc). Then use "Windows Server Backup" to specify this local drive as destination type.
※ Hotfix: Microsoft hotfix KB2920193.