Here is my issue. I have a working a Jetson Nano 4gb dev kit, that is working fine so far.
I acquired a new one, then cloned the SD card and it bricked the new dev kit.
I acquired a third one, first flashed a new SD card, it was working fine, then uses the SD card of the initial dev kit, and it bricked the third dev kit.
“Bricked” means it is unrecoverable, but this is only a software issue. The background you are missing is that a dev kit has onboard QSPI memory which is used for boot. Older releases have different QSPI content compared to newer releases. If the SD card content does not match (in terms of release) the content of QSPI, then it will fail. There might have been some attempt to update the SD card when placed in a unit with the wrong QSPI memory. Likely each unit has different QSPI content.
The fix is to flash the dev kit itself to use a release compatible with the SD card image. You would need an Ubuntu 18.04 host PC and flash with JetPack/SDK Manager. That SD card might work then, but it might also need to be flashed as well depending on circumstances.