A "BAK BATTERY LOW" message in the display is an indication that the internal battery is running down in the DR-670. The internal battery is designed to save your custom/user settings when the power is turned off.
The battery is a model 2032 (watch/calculator type) and can be found at most all electronic supply stores that have a battery section.
The internal battery is not considered to be "user" replaceable since it is mounted on the main circuit board and the DR-670 would need to be disassembled to access it. Due to safety reasons, we recommend having it changed by an authorized technician to avoid any damage to the DR-670.
You can use our Service Center Locator at http://www.rolandus.com/support/service_centers/ to locate an authorized Roland Service Center in your area.
Note: While it will not cause harm or permanent damage to the DR-770 if the battery runs down, ALL User settings will be lost when replacing the battery. Be sure to backup any user settings you wish to keep before replacing the battery.