A nonrecoverable I/O error occurred on file "" 64(The specified network name is no longer available.).
You have scheduled your backups to a Shared Location or Network Path. Your backup was running fine, one fine day you suddenly come across an error saying "A nonrecoverable I/O error occurred on file "" 64(The specified network name is no longer available.)."
When you check for more information in logs you will find more description such as
"Executing the query "backup database  to disk='\\..." failed with the following error: "A nonrecoverable I/O error occurred on file "\\NetworkPath\database.bak:" 64(The specified network name is no longer available.).
BACKUP DATABASE is terminating abnormally.
Processed XXXXXX pages for database '', file '' on file 1.
Processed X pages for database '', file '' on file 1.". Possible failure reasons: Problems with the query, "ResultSet" property not set correctly, parameters not set correctly, or connection not established correctly."
For Example lets say there are 10 databases on your Instance.
You come back and see that on your Network path is working fine. You are able to access the Network Path and also can see the backups existings for some of the database.
The connection is perfectly working fine.
So what went wrong and what is this error then, what will I do to fix this. Well there is nothing to fear and worry until this is happening frequently in case you need to work with Network Team to resolve Network Glitch.
What exactly has happened is when a backup is running, there might be a Network Glitch or the Network from SQL Server to Network Path have might been broken for few moments. The error itself says it "The specified network name is no longer available."
SQL Server isnt that good at handling Network Glitch and Retrying, a very small Glitch is overcomed, however if the Database Size is huge and there is a Network Glitch chances are more that SQL will not retry for long time and it fails the backup.
A simple solution is to Re-Run the Job or just take a Adhoc Backup only for those Databases which have not been backed up, also you could take all the backups on to Local Disk and then copy these to Network Path using some Robost copy softwares such as ROBOCOPY, which handles Network Glitch in a better way.