Repair unmountable external hard drive with Disk Utility built-in Mac OS X
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:
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.
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 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.
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 "In a specific volume" or "In a specific hard drive" option and then choose the unmounted external hard drive from the drop-down drive list.
Step 3: M3 Mac Data Recovery is scanning 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.