AJ -
We had the same problem with our old Designjet 430. Both OS9 and OSX were completely unsupported by HP. We had to fork out for 3rd party software for OS9 and the Gimp driver for OSX resulted in very poor quality plots.
I've had a look and there is a HP driver for the 500 for OSX, but for 10.4 it says:
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HP Printers - Mac OS: Print only HP Drivers (non-Postscript) built into Mac OS X v10.4 (Tiger)
Mac OS X v10.4 Tiger and HP Driversback to top of page
The latest HP printer driver for this device is built into Tiger for print only functionality and does not require a download or reinstallation of any HP software to print.
This document applies to raster (non-Postscript) HP Designjet, Deskjet, Business Inkjet, and LaserJet printers.
In order to best take advantage of the latest HP driver and new Mac OS X v10.4 Tiger features, it is recommended that you delete original HP printer queues from Printer Setup Utility and create a new HP printer queue.
Tiger - HP printer setup instructions:
1.Upgrade install from Mac OS X v10.1, 10.2, 10.3 - select 'Reset Printing System" option (from the Printer Setup Utility pull down menu) to delete all previous printer queues. The HP printer queue can also be deleted manually from Printer Setup Utility.
2.From Printer Setup Utility, click on Add, select connection type (e.g. All, Bonjour, USB, Appletalk) and create new HP printer queue. The Printer Setup Utility is found in the Applications/Utilities folder.
3.Clean install - If there is a clean install of Tiger, follow step 2 to add a new HP printer queue.
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That has me scratching my head because as I said, the 500 isn't listed in Printer Setup.
Any ideas?