How to recover a corrupted external exFAT hard drive
I think the drive is already unmounted, if not then unmount it and do this 

try mounting in read only mode by doing this 

diskutil mountDisk readOnly /dev/disk2s1 


Check if the drive is hijacked by the system

ps aux | grep fsck


If you see your disk there hijacked then use this to kill this process

sudo pkill -f fsck


You should immediately see your drive at the finder's sidebar.