macOS: Can't mount exFAT External Drive
If your ExFAT drive is not mounting, here is the solution.

sudo pkill -f fsck


fsck was holding the disk hostage. A quick ps aux | grep fsck revealed that indeed it was hijacking the disk/volume as soon as it was plugged in. sudo pkill -f fsck (or just kill with the PID if you prefer) immediately allowed the volume to be mounted.

Then to make the hard drive writable, you have to run the Disk Utility "S.O.S.", unmount then mount it again.