Brother Copier/Printer MFC-9970CDW - Manual (Page 122)

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Toner Low
The toner cartridge is near the
end of its life. Buy a new toner cartridge for the color that is
indicated on the LCD before you get a
Replace Toner
message.
Too Many Files
There are too many files stored
on the USB Flash memory drive. Reduce the number of files stored on the USB
Flash memory drive.
Unusable Device
An incompatible or defective
device has been connected to
the USB direct interface. Remove the USB Flash memory drive from
the USB direct interface.
Error Message Cause Action