External hard drive not mounting in Mac, how to recover data and fix it?

Since updating to macOS Sierra, my Seagate 3TB external hard drive is not mounting by macOS High Sierra 10.13/10.12 Sierra. In Disk Utility I cannot do anything but format it which I don't want to do, because there are many important files stored on it, any suggestions to repair unmountable disk or recover data from unmountable external hard drive?

external hard drive not mounting in Mac disk utility

Solution 1: Repair unmountable external hard drive with macOS built-in Disk Utility

Although Mac OS X built-in Disk Utilitiy has the ability to repair unmountable external hard drive, but in some situations, Disk utilily cannot repair this disk. Back up as many of your files as possible, reformat the disk, and restore you backed-up files:

Disk utilily cannot repair external hard drive

In this situation, data recovery from unmountable external hard drive is the best way. After data recovery is done successfully, use the disk utility to reformat the unmounted external hard drive, then the unmountable external hard drive will be working again.

Solution 2: Recover data from external hard drive not mounting in Mac

M3 Mac Data Recovery is a professional Mac data recovery software which can recover data from external hard drive not mouting in Mac computer, recover deleted files even if emptied from trash, recover data from unreadable, formatted, corrupted drive in macOS High Sierra 10.13/Sierra 10.12 and Mac OSX 10.11/10.10/10.9/10.8/10.7 and supports HFS, HFS+, exFAT and FAT32 file systems.

Tutorial to recover data from unmountable external hard drive with M3 Data Recovery

Step 1: Download, install and then launch M3 Mac Data Recovery on your Mac computer.

Step 2: Select the volume or external hard drive

Free Mac data recovery software - M3 Mac Data Recovery

Step 3: M3 Mac Data Recovery is scanning lost data from unmountable external hard drive

Scan lost data from unmountable external hard drive

Step 4: Select the found files you expect to recover and then click Recover to recover lost data from unmountable external hard drive.

Recover data from external hard drive not mounting in Mac computer

After data recovery from unmounted external hard drive is successfully done, please try to reformat it, it will be working again after format completes.

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