I’m disassembling yet another iMac this evening that someone here used Target Disk mode on (corrupting the SSD firmware) after I just went through this & forbade TDM last week. Luckily it’s old enough that the drive is replaceable. Bc AppleCare expired 3 months ago, Apple offered a replacement 1TB (Samsung) SSD for $700 lol. On the plus side, it’s a good opportunity to bump it up to a 4tb aftermarket SSD for $500 instead, with the added benefit of Monterey not being able to be installed on it ever again. Mojave forever.
The Genius Bar techs were familiar, confirmed they’ve been seeing a lot of this, & narrowed it down to the changes in how Monterey handles higher level aspects of the file system. In looking up the specific errors of this problem I found quite a few others having to shoulder the costs of new SSDs & Logic boards.
Monterey has a big problem.