[someonehere]

Gut feeling says these are older computers where the drives are user replaceable. I’m almost willing to bet the firmware assumed the stock drive from Apple was in it but those users put larger platter or SSD drives in those machines. Firmware attempted to flash and prep the stock drive to APFS for Catalina (they possibly were not on APFS and on 10.13 with HFS+), and low and behold they’re bricked.

I’ll Bet $20 if they put a stock Apple drive and tried again it’ll work. When I worked Apple retail we would decline repair of machines that had the hard drive or RAM replaced with a third party manufacturer. These machines are out of warranty, Apple won’t likely give them the time of day unless they pay out the nose.

[evenh]

I have a friend with a 13" MacBook Pro Retina (Early or Mid 2015) which this happened to. Nothing has ever been modified on that computer, hardware wise.