Identify problem with Network / NAS copying 20GB data with Robocopy to External HDD

He.
We have a 8TB WD Sharespace NAS which we save full backups onto over the weekend and incremental backups to between 9am-5pm weekdays.
We need to transfer the most recent Full backup and all incrementals since then to an external HDD each weeknight for offsite DR.

I've setup Robocopy with /maxage:7 switch which apparently should be able to transfer files hundreds of GB's in size reliably and consistently.
However during our test backup last night we got several errors inconsistently on a number of the larger files (there should be minimal network activity during weeknights, and no overnight backups):

This error once on a 20GB file before it succeeded:
ERROR 64 (0x00000040) The specified network name is no longer available.
Waiting 5 seconds... Retrying...

This error 3 times on a 158GB file before it succeeded:
ERROR 64 (0x00000040) The specified network name is no longer available.
Waiting 5 seconds... Retrying...

This error once on the same 158GB file before it succeeded:
ERROR 1130 (0x0000046A) Not enough server storage is available to process this command.
Waiting 5 seconds... Retrying...

This error 5 times on a different 159GB file before it succeeded:
ERROR 64 (0x00000040) The specified network name is no longer available.
Waiting 5 seconds... Retrying...

This error once on a 9GB file before it succeeded:
ERROR 64 (0x00000040) The specified network name is no longer available.
Waiting 5 seconds... Retrying...


All other files (71 files ranging from a few MB's to 69GB) copied across fine the first time..

How could we diagnose the problem (likely either hardware or network related)?


PS: This is the complete script:
del /s \\server\SPBackups\*.*
robocopy \\wdsharespace\\SPBackups\ \\server\F$\SPBackups\ /maxage:7 /z /e /log+:C:\NAS_Backup\robocopy_log.txt /w:5 /r:20 /np /purge