The specified network name is no longer available

[satac]

The following command (from a Windows machine) gives the error "The specified network name is no longer available." (where "sqlbackup" is a “network share” offered by the QNAP)
Can anyone offer any help on why this is occurring? The information that may be relevant is below:

c:\sqlbackups\> copy VeryLargeFile.bak \\qnap\sqlbackup\ (We don’t use a mapped network drive letter) Very Large File size is about 50gb

This error occurs quite some time after the copy command is started. The copied file on the QNAP grows, and grows, and nearing the copy completion, the above error occurs, and the destination file is deleted.
The QNAP connection log has two entries,
Connection Type Accessed Resources Action
SAMBA sqlbackup\VeryLargeFile.bak Delete
SAMBA sqlbackup\VeryLargeFile.bak Add
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When small files are copied, this command will work, and the file will be copied. However, for large files, the copy fails.
Further to this, this command *will* work for large files from one Windows 2012 client, but not from another Windows 2012 client. (both are Hyper-V Virtual Machines)
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Both VMs have up-to-date Network adapter drivers, (from the "Integrated Services")
The Version of the one VM that can copy to the QNAP has Version 6.3.9600.18339
The Version of the one VM that can NOT copy to the QNAP has Version 6.3.9600.18338
The QNAP have its two NICs bonded/teamed to one IP address.
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Jumbo Frames are enabled on the QNAP, and on the two VMs using the command below:
C:\>ping qnap -f -l 1472 (See http://www.tp-link.com/us/FAQ-190.html)

Pinging qnap [10.1.1.18] with 1472 bytes of data:
Reply from 10.1.1.18: bytes=1472 time<1ms TTL=64
Reply from 10.1.1.18: bytes=1472 time<1ms TTL=64
Reply from 10.1.1.18: bytes=1472 time<1ms TTL=64

Ping statistics for 10.1.1.18:
Packets: Sent = 3, Received = 3, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms
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We have a QNAP Model: TS-879U-RP
The QNAP Firmware Version is: 4.2.2 Build 20160812