This problem is caused by system utility fsck which is abbrevation of File System Consistency Check, a tool for checking consistency of a file system in Unix and Unix-like operating systems, such as Linux, macOS, and FreeBSD. This Solution is based on This StackExchange Question but with detailed steps.
sudo pkill -f fsck
Kill fsck process. After doing this, Mac will give you a warning as following, just ignore it. Now you may see your external drive in FINDER.
Use DISK UTILITY App macOS provide, Click FIRST AID on your external drive. This may take some time, after this, the problem is solved.
This Sulution is working on macOS Mojave 10.14.6. The external drive is ExFAT format.