Networking - Networking Basics - Discussion

Copy running-config startup-config command used only save the changes into switch/router NVRAM

while copy running-config TFTP command is used to create and save this backfile in TFTP server.

The command copy running-config tftp will not work because you don't have written permission on the TFTP server. The version of 'copy' that comes with IOS uses a proprietary transport layer to get data onto a server. That transport is called TFTP. TFTP is not a reliable nor persistent transport and doesn't require user account credentials.

It seems the OP has the right idea but the wrong command which is the copy tftp running-config command. This command will copy all files in flash to the TFTP server and delete files that are not present on both devices.