Metal square over 3.5" drive head fell off, is it required?

I would glue them back.

I believe those are mu-metal to provide magnetic shielding for the heads. I have seen them fall off as well.

It should be straightforward to glue them in using a drop of CA adhesive.

jasonnovak wrote:

Thanks, I'll super glue the top one back on, bottom one looks like have to take it all apart and probably mess up the alignment or something

Difficult to perturb the alignment on these drives unless you mess with the position of the stepper motor on the end.  You can easily remove the read-write sled and access the bottom of it.

This video shows removing the sled around 47 minutes. The 800k drive chassis in macs are the same as the IIgs. You will also want to clean and regrease the eject mechanism if you haven't. And replace the eject gear. If it hasn't disintegrated yet, it will.  

https://m.youtube.com/watch?v=sy6JWEerHQc

I'm not a fan of using the super lube he shows in this video. Especially when he talks about having to thining it out with oil. He does mention molykote em30l though. That's actually what I use and would recommend. I'm also not a fan of his recommendation to use wd40 as a cleaner. Just stick to 99% ipa. 

Agreed, although using only isopropyl will make technical cleaning more challenging. It helps to have a wide range of solvents/degreasers on hand to deal with different types of soils. The least amount of labor is to use a vapor degreaser or ultrasonic bath.

A NLGI 2 grease is a bit too thick for this application. NLGI 1 would be ideal. Higher viscosities are needed in gearboxes under higher loads to prevent grease from being flung away from the gear teeth, so something like Super Lube might be appropriate for the gear teeth in the eject motor assembly (which are not lubricated at all in the video). But a molybdenum disulfide grease would likely perform better for that.

A point that is sometimes missed is that many 3.5" drives have SMD electrolyic capacitors which are becoming one of the most common reasons for failure.

Can't speak to third party drives, but the Apple 800k (and 400k) drives don't use SMD electrolytic. They use radials. I can't say that out of the seven 800k and two 400k (which those eject mechanisms are a pain to service compared to the 800k) I have worked on, I haven't seen any signs of leakage when I cleaned/greased/recapped them. But at least now I know they will out live me. 

That prompted me to tear one of these drives apart, and there's no plate on the bottom of my head either.  In fact I can't see where that second square might have come from.