Upgrading raspbian from jessie to buster.
Stop all processes you can without locking yourself out of the pi.
Use rsync --delete-before --delete -aHAXx to replace all files on the running system (one partition at a time). Replace files on the root partition last.
Remount filesystems read-only and reboot the pi:
echo u >/proc/sysrq-trigger
echo b >/proc/...
This looks like the end of a kernel panic message. It's hard to say what caused it without seeing the full message, and even preceding messages, say, 10 seconds before the crash. The fact that one of the CPUs was inside fat_write_begin when the crash happened doesn't mean the crash had anything to do with the filesystem. It's impossible to determine from ...
Yes, remove all additional hardware first.
If that doesn't help, try cloning the contents of your SD card to a new SD card. If the error no longer occurs, then it was due to a broken SD card.
Otherwise, you could reinstall the operating system (possibly make a backup of the existing OS first).
If all this does not help, then your Pi seems to die soon. Maybe ...