Disk mounting error

Open Disc Utility and select the drive. Run First Aid.

Another option is with Terminal. Open Finder > Applications > Utilities > Terminal.

Type the following command to show all the drives on Mac.

Then, find the identifier of the unmountable drive.        

diskutil list

Type the following diskutil command to mount the drive. Here the disk1s2 should be replaced with the identifier of your unmountable drive.        

diskutil mount /dev/disk1s2

The third option is:

• Open Activity Monitor > search fsck in all tabs (CPU, Memory, Disk, etc.). 
• When finding the fsck, select it and click the × icon in the upper-left corner. 
• Select Force Quit.

The information is form iBoysoft article.

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https://developer.apple.com/forums/thread/680500