I have an old HP printer & an old e-mac, but, apart from the slowness, no problems.    Printer:  Hewlett Packard 5740 printer.   I have 2 HP cartridges in place - a #97 lge. size color cartridge & #97 BLACK cartridge.  I have been printing, the cartridges are not low on ink. - Now suddenly I am getting a blinking light on my printer & a printer message on computer saying that the black ink cartridge is not compatible with my printer.  I have taken it out, reinstalled it - still the same message.  I have been printing with it right along - now this message & there is seemingly no way to over ride it.  the cartridge is fine, it the right model # 97 - I've been printing with it!  Any help appreciated - short of giving up & buying a new printer, I don't know what to do.  This is crazy!There are a great number of options for you to try and overcome this issue. I accept that ALL the equipment involved is unchanged and has worked properly in the past. The easiest attempt would be to replace the black ink cartridge and see the result. The next would be to take the black cartridge to an ink refilling station and have them reset the microchip (if it has one). You can also TOTALLY uninstall the printer hardware/software and reinstall it. This method should ONLY be tried if you have the manual and the original software installation DISK. Or if you can find both as available online. Because as if you say the printer is old finding the appropriate drivers for it  online now may not be possible. Without going into DETAILS (because for some reason most ink jet printer users are intimidated by the process) you can also purchase the materials to replace the cartridge/chip and bulk ink/hardware to do all this yourself. You need to be aware that any costs to try and resolve this issue with the same printer should be weighed against the purchase of a replacement. Printers by in large are almost FREE in order to sell the cartridges. If you do go for a replacement be careful to ensure that it will be compatible with your Apple computer model.truenorth