Your intuition is a bit off - that's what gives you most of the troubles. After you have formatted the drive there is nothing left over - i.e. you do not have to worry about "buffer files from exfat" - cannot happen.
Similarly the diskutil listing is in no way any indication of "earlier attempts" or parts of a "7-pass format" - doesn't exist, cannot happen....
Will I also run into these issues when I just remove the HDD?
The reason being is because that vintage iMac used a quasi-proprietary drive that had a temp sensor built in. Remove the drive, you remove the sensor and the fans spin up full speed.
Instead of just removing the drive, add an SSD and use an inline temperature sensor to regain the ...
It depends on the app:
Non-Apple applications states or data can be found in ~/Library/Application Support/ usually.
Copying the folder named as the app to the respective path in your new system should restore most things.
This works for Firefox and Chrome.
It's also adviseable to copy the content of ~/Library/Saved Application State/.
Some Apple apps ...
Seagate provides an OEM-version of Paragon's Microsoft NTFS for Mac. The software is availbale at: Paragon Driver for macOS (10.10 and above). Direct link: NTFS_for_Mac
The installer build date is Sep 12, 2019. The installer's info.plist indicates compatibility:
After downloading and installing it (and probably rebooting your Mac),...
I'd suggest putting both the harddisks back into the devices they were originally in and try booting again. Make sure the laptops are plugged in when you do this. If that doesn't work, it's very possible you've damaged the disks by trying to pull them out while the laptops were running.
If it does work, then I'd use time machine to create a backup of the ...
The broken header appears to be the right speaker connector (or the sleep LED connector).
Comparing it with the "left side" (and considering the two cables only) I tend to the speaker connector. The accompanying text is not very precise.
Both links/pics from: iFixIt's MacBook Unibody Model A1342 Logic Board Replacement
Okay, I can answer by myself.
In further investigation I came across the hidden space called local snapshot space that is caused by local time machine snapshots and tried to purge them for giving back at least 100 GB:
♠ sudo tmutil thinlocalsnapshots / 100g 1
Thinned local snapshots:
A simpler yet still pretty effective approach is to install macs Fan Control
in there you can easily set you fan speed to be controlled by any other sensor (just pick one that is close enough to the main source of heat)
voila, fan speed down, no additional hardware tinkering and works very nice with SSD as well