I'm stupid.
I stupidly threw away a Kodak printer because I couldn't get it out of a "3527 Printhead jammed" error. I saw a broken piece on a drive belt and ass-u-med that was the problem. It may have been A problem, but it wasn't THE reason for the printhead blockage.

The correct fix is shown in this clip, which has been around for a while. I even watched this clip while looking for solutions, but didn't understand EXACTLY what the guy in the clip was saying.

I tossed the old printer and bought a "new" used one. The "new" one came today, and as soon as I turned it on, it showed the SAME DAMN ERROR. I didn't feel like tossing yet another printer, and figured that this one was not likely to have the same missing bit on the belt.

So I googled everything again and watched that clip again, after opening up the inner cover and taking out the printhead. This time I realized EXACTLY what the guy in the clip is saying. He's not really showing the innards, so the visual aspect is lacking.

Let me try to add something to what he's saying, to focus tightly on the EXACT problem. Maybe this will help others who ass-u-med the same thing that I ass-u-med. (Probably nobody else is equally stupid, but just in case!)

The critical item is the one-inch square that is exactly centered inside the printhead holder. This square looks like it should be there; it's popped up exactly in the place where the printhead fits. Because it's popped up, it's preventing the carrier from moving out of this location. The problem is that the one-inch square SHOULDN'T be popped up, and it SHOULDN'T be exactly in that place. You need to use a screwdriver to hold the square DOWN far enough to clear the carrier. This isn't easy because the square sort of wiggles around. While holding DOWN, simultaneously use a finger or a second screwdriver on the front edge to push the square STRAIGHT BACK. This doesn't seem right; the square acts like it's not supposed to go backwards; but this IS right, and the square DOES obligingly move back when you push it.

Immediately put the printhead back in the carrier, to prevent the square from popping up again. Immediately load cartridges in the printhead, then close the inner lid. Don't bother to screw it back down; it's much easier to fiddle with the innards when this lid remains loose. Close the outer lid and turn the printer on. It will then run through a normal startup routine. It will ask to have the "new" printhead calibrated, which is normal because you have in fact put a "new" printhead in. Go ahead and do that, then you're ready to print.

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Looking back on both situations, I think this problem happens with a rather old printer with 'loose joints', when you operate or ship the printer without cartridges in the printhead. Without the carts, the printhead isn't heavy enough to keep the one-inch square from popping into its locked and upright position. Good advice (for stupid me anyway): Keep cartridges in the printhead at all times. Better advice: Buy a brand-new printer. Think I'm going to do that.

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Later: I did buy a new printer, but not a Kodak. Went back to HP. No more trouble.
  ¶ 4:00 PM