No, they shouldn't have to be running the same OS & using the same driver. At my plant, the 7475 was attached to an HP Jetdirect port, not an actual computer. There were several 98 & NT4 machines hitting it, along with the one XP Pro workstation.

Still, I wouldn't think that the machines using different drivers would cause a problem. Like TripleR said, were there any error messages? The plotter act like it was receiving data at all? Or did it just do nothing?