So it's either that the two read/write heads in the Plus's drive are misaligned, and so only the lower one works when it's alone (hence the 400ks being read but not the 800ks)...or else my SE fudged up the disks.

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So it looks like my theories are well-founded after all...

NEVER stick anything in an untested drive you don't expect to get trashed.

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Yeah, I'm aware of that. What can I say, I'm a risk taker!

But I'm not too worried; I'll do more experimentation in the future and I'll figure all this out.

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Tonight I'll turn the Plus on again and do some file-saving experimentation with 800k disks, seeing if the Plus can read its own disks, and whether the SEs can read the disks the Plus makes. I'll pinpoint this issue!

And then I'll unplug it, wait a few days, and then open it up to check for any capacitor leakage. I don't want any corroded traces...

-Apostrophe