If you ensure the WD USB exFAT drive is not the one to blame given that it works well with Windows PC. Then, to fix WD external drive that won't mount on the Mac mini, you can try to check the connection, check for driver updates, reset SMC, disconnect unnecessary peripherals, and uninstall recently installed apps.
There are multiple reasons for external hard drive not mounting on Mac such as damaged file system, poor connection, defective USB port, outdated disk driver, system errors, etc. If you can use the WD drive on Windows without any problem, then the issue should be with your Mac mini.
First, check the connection between the external drive with your Mac when WD external hard drive won't mount on Mac, defective USB port, USB cable, and USB hub could be the culprit of the mounting issue.
After making sure there is no connection issue, try to manually mount the WD USB exFAT drive in Disk Utility by selecting the drive name on the left sidebar and clicking the Mount button on the toolbar.
Now, you can try to reset SMC if the WD external drive remains unbootable, and updating driver for external drive by updating macOS or download software from the official website may help to fix the problem.
If you still can't mount the WD external drive on Mac, you can format it so that you can use it on Mac without any problems. This process will deletes all contents stored on the drive, you'd better back it up on Windows in advance.
In case any data is lost during the process, free download iBoysoft Data Recovery to recover data!