A drum unit is a maintenance item for your Brother printer or other manufacturer Laser printer that will sometimes need to be replaced but not as often as a toner cartridge. A drum unit is a green metal cylinder or tube that carries a static electrical charge that attracts the toner powder from the toner cartridge and applies it to the paper as it passes through the printer. Therefore, you will always find that drum unit is located very close to or even attached to the toner cartridge.

How do I clear the message ' Drum Error '?
Resolving the “Replace Brother Drum” message
When the "Image Drum Near Life" message appears on the display screen, prepare a replacement image drum. If you continue printing, the message "Image Drum Life" will be displayed and printing will stop. When the "Please install new Image Drum Unit. This estimation assumes the standard use condition three pages are printed at a time. Printing one page at a time reduces the drum lifetime approximately by half. The actual number of printed pages may be less than half the above estimation depending on the printing conditions.
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The Brother drum unit is a separate consumable that typically should be replaced after the use of 3 or 4 toner cartridges. If you own a Brother laser printer and you are getting a Replace Drum message on your machine's display, it might be time to get a new drum. Your printer uses a page counter to track usage, which must manually be reset every time a new drum unit is installed.
Developed and used by cultures living in forested areas, drums served as an early form of long-distance communication , and were used during ceremonial and religious functions. While this type of hourglass -shaped instrument can be modulated quite closely, its range is limited to a gathering or market-place, and it is primarily used in ceremonial settings. Ceremonial functions could include dance, rituals, story-telling and communication of points of order.