I/O errors backing up to an SMB Share from MSSQL Server 2014
Just following up on this. Backups have been running flawlessly since we made a change to the SMB timeout in the registry:


[HKEY_LOCAL_MACHINE\\SYSTEM\\CurrentControlSet\\Services\\LanmanWorkstation\\Parameters]  
"SessTimeout"=dword:00000258


What that works out to is 600 seconds or a 10 minute timeout for SMB sessions from the SQL Server, the default is 60 seconds. it needs to be saved in hex.

I found it in the article: CIFS and SMB Timeouts in Windows

It's the line with a heading of Client Session timeout.

We arrived at this solution after verifying in a packet capture that the session was being ended by the SQL Server sending an RST, ACK to end the session.