The toner cartridge is only a supply chamber. The cartridge simply dumps toner into the feed hopper in the drum. When you change the drum the feed hopper is empty and it is likely that the cartridge dropped the last of it supply into the hopper - ergo it is now empty. I recommend shaking the toner back into the cartridge before separating the cartridge from the drum unit when changing them
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The toner cartridge is only a supply chamber. The cartridge simply dumps toner into the feed hopper in the drum. When you change the drum the feed hopper is empty and it is likely that the cartridge dropped the last of it supply into the hopper - ergo it is now empty. I recommend shaking the toner back into the cartridge before separating the cartridge from the drum unit when changing them