When you get the error message "Slide the Green tab on Drum Unit?" after replacing a new drum unit, please perform the following steps.
1. Take out the drum unit and toner assembly.
2. Clean the electrodes with a soft cloth.
Printer inside - right side of the toner cartridge compartment
Right side of the drum unit
3. Gently slide the green tab from side to side several times, and return it to the home position at the far left.
4. Re-install the drum unit.
5. Repeat above steps for all other color drum unit.