Mac workstations running OS X 10.10/10.11 connect to SMB using the SMB 3 protocol by default, and Mac workstations running OS X 10.9 connect to SMB using the SMB 2 protocol by default. Sometimes these newer SMB protocols will fail to work when Macs attempt to connect to NAS devices or newer Mac servers.

The SMB connection method can be changed by modifying /private/etc/nsmb.conf. If this file doesn’t exist, create it first by entering this into the Terminal:
sudo touch /private/etc/nsmb.conf

To modify Mac workstations running OS X 10.9/10.10/10.11 to connect using SMB 1, edit the /private/etc/nsmb.conf file using TextWrangler. Unlock when prompted, then modify this file so it only contains these two lines:
[default]
smb_neg=smb1_only

To modify Mac workstations running OS X 10.10/10.11 to connect using SMB 2 instead of SMB 3, edit the /private/etc/nsmb.conf file using TextWrangler. Unlock when prompted, then modify this file so it only contains these two lines:
[default]
smb_neg=smb2_only

Restart for these changes to take affect. To restore a Mac to normal SMB protocol connection behavior, delete /private/etc/nsmb.conf and reboot.