DesignJet, Large Format Printers & Digital Press

I have the identical problem that karencpafla is experiencing.  I have already set up a custom paper size of 9 inches by 12 inches in my MacBook's page setup.  When I try to print with  9 x 12 paper (a common size in my music work), I get this error:

     "Paper size mismatch. The paper size detected by the printer sensor does not match the paper size selected in software. If the paper loaded in the printer is the correct size, change the selected paper size in your software and re-send the print job. If the paper loaded in the printer is not the correct size, change paper and resend the print job."

I have already tried all of the recommendations mentioned in karencpafla's thread and in other threads on this same topic.  Nothing worked.  I just reinstalled my printer drivers (for the third time) and tried resetting the printer by unplugging it while it was idle.  Still no success.

In another thread ("cannot print on 9x12 paper from Acrobat"), the HP tech Rainbow 23 "resolved" the issue by saying the following:

     "I understand that the issue still persists after following the steps provided in the previous interaction.

     "This issue has occurred because the paper size that you want to print is ARCH A size. This paper size is only supported by high-end printers like HP Designjet and HP Plotters. "

That "solution" makes no sense.  For years, I printed on 9 x 12 paper using my HP OfficeJet Pro 7500A and never had a problem with that paper size.  When my 7500A died recently, an HP phone tech told me the 7500A is now obsolete and has been replaced by the 7740, which I then purchased.  Why would HP remove the ability to print on 9 x 12 paper from its OfficeJet Pro wide-format printer??

I am hoping there is a glitch with the printer driver or the firmware that can be corrected.  Can you please check into this as my music printing operation depends on being able to print on 9 x 12 paper.  If HP truly no longer has a solution for printing on 9 x 12 paper, I will need to return the printer and purchase another brand of printer that can.

Thank you for your help.  If karencpafla or any other user has resolved this issue, please chime in, thanks!