I have Brother MFC8460N and right now does not print at all, dust on drum message is appearing. Please help

If it still prints ignore it.
if you rather call the vendor. Ifyou're under warranty. They may replace or repair it.

Print Unable 71 means Polygon Mirror Failure on the Brother HL-4040, MFC-9840 and MFC-9440.
Pretty safe to say it means the same thing on the MFC-8880.

The Polygon Mirror is part of the Laser Unit, and its rpm isn't within specs.
The Laser Unit must be replaced.

Hi there
Not know it you got the mauaul.
I have put a PDF files on my website for you to download.

Just click on the links it will download for you.
It got all you need to know in this file.
You can read it on site and print it strait off the website or save it to your PC then print it out.

Click LINK= http://www.4all2sell.com/pdf/Brother-DCP-8060-MFC-8460N-Manual.pdf

You will need a adobe reader to see this file If you not got an Pdf reader you can download it off this link free.
http://www.adobe.com/products/reader/

Please come back and just gave a vote, it will gave me some satisfaction to know am helping some one so I will keep on helping.
All The Best: Pineworks

Try this:
2.3 Drum Unit Cleaning Mode
This mode is to remove the dust attached on the exposure drum.
When white spots appear on the printout, perform this operation to
remove the dust by using the drum cleaning sheet.

<Operating Procedure>

(1) Load the drum cleaning sheet into the MP tray.
(2) Check that the status LED lights in green and press the and
Clear/Back buttons at the same time. “DRUM CLEANING” will
appear on the LCD.
(3) Press the Black Start button. The machine starts cleaning the drum
unit. “DRUM CLEANING / PLEASE WAIT” appears on the LCD
during cleaning.
(4) When finishing cleaning normally, “DRUM CLEANING / COMPLETE”
appears on the LCD.
(5) Press the Stop/Exit button so that the machine returns to the ready
state.

you need to reset the life of the drum. Instruction is included when you purchase a replacement drum

That could be from your fuser unti

Well depending on what it looks like it can be caused by a variety of issues. Because it's a laser printer fixing it can be difficult. Here is a link to an excellent troubleshooting/repair guide for laser printers. I hope this helps you.
http://www.printertechs.com/tech/print-defects/print-defects-index.php

The dust on the drum message will persist if the corona wire cannot be cleaned by sliding the blue tab repeatedly left to right. You'll need to replace the drum unit.

You need to change the drum on your printer. Part # DR-350.

Thanks and please take a moment to rate as fixya.

This message would appear if the drum unit is near the end of its life. Use the drum unit until you have a print quality problem, then replace the drum unit with a new one.

This message would continue to appear if the clear plastic starter sheet was removed from the "new" drum before it was installed. This plastic starter sheet, resets the drum counter. To reinstall drum with the starter sheet, please follow the instructions listed below:

Locate the starter sheet. Clear plastic sheet, 9 inches long, with a cut out in the center. The sheet will have "Initialization of your new drum successfully completed. Please discard this sheet" written on it.

If you cannot locate the starter sheet, contact the Brother Customer Service or the appropriate Brother authorized dealer.

additionl info HERE