In this situation, external hard drive has been corrupted, Mac operating system fails to mount it and we cannot access the data stored on it.
There are several common reasons which cause Mac hard drive fails to mount, such as file system corruption, volume header corruption, disk Utility failure, Log file corruption, OS malfunction, sudden system reboot. How to mount an unmountable drive Mac and access the data? There are several ways, but the best way is to recover data from unmountable drive and then reformat it with disk utility, then the unmountable drive will be mountable again after format completes.
Repair unmouuntable drive in Mac
Disk utility has the ability to repair unmountable Mac drive, it has a built-in function which called "First Aid", Fist Aid can check the volume for errors and then repair the volume if necessary. After reparing with disk utility, the unmountable drive will be mountable, but sometimes, disk untility cannot repair unmountable Mac drive. In this situation, the best way is to recover data from unmountable drive and then reformat it.
Recover data from unmountable drive in Mac
M3 Mac Data Recovery is the professional Mac data recovery software which can recover data from unmountable hard drive, external hard drive, USB drive, flash drive, etc. in macOS 10.13 High Sierra/10.12 and Mac OSX 10.11/10.10/10.9/10.8/10.7/10.6 and supports HFS, HFS+, exFAT and FAT32 file systems.
M3 Data Recovery can also recover deleted or lost files, recover data from formatted, inaccessible, corrupted hard drive, external hard drive, USB flash drive, memory card, SD card, etc.