Typically on Cisco IOS the copy command will use the default management interface for TFTP traffic. After quite a bit of throubleshooting I found out this is not the case with the NX-OS. You need to put in a vrf. By default for the management interface ‘management’ must be entered.

Nexus_5010_Switch# copy tftp:// bootflash
Enter vrf (If no input, current vrf 'default' is considered): management
Trying to connect to tftp server......
Connection to Server Established.