HP Officejet 6400 drivers for macOS 11 are not available from the HP website or Apple Software Update. However, Gutenprint (formerly called Gimp-Print) provides open-source basic print drivers for many printers that would otherwise not be supported in Mac OS X. To see if your printer is supported, go to Gutenprint drivers for more information.
NOTE: Gutenprint drivers are not supported by HP or Apple.
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Thank you for your help. Gutenprint does not have an OJ 6400 printer driver either. At this point I have updated 2 of my Macs to 'Big Sur' but will not do the 3rd one until I find either a compatible driver or a replacement printer.
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