Sorry, I responded to the wrong post with this answer. I was answering two at once. The spots are usually caused by toner build-up on the wiping blade of the drum. It clumps. Again,if you are using non-OEM toner, the formula is different. At the company I work at, we manufacture toner in addition to selling OEM. Our national sales team actually recommends OEM toner to those clients who purchase the smaller HP units. The larger units are easier to take apart and clean.
The smaller units, not so much. Anything with "Pro" in it's name was designed and prototyped by the same folks who develop the ink printers. They are used to working with smaller, robotically built machines.
Magenta has ALWAYS been the worst. I don't know if they still do it, but in the older units, the engineers actually designed the magenta portion of the power supply to shunt the excess charge off the drums of all colors. That was why Magenta was always the messy one.
As far as the black spots go, again, the formula is not the same and reacts differently under heat. Even the way it flows. It's like anything else... generics are an attempt to copy the original.
