To fix your external hard drive not mounting on your MacBook, you can recover your data from the unmounted external hard drive first. iBoysoft Data Recovery for Mac can help you to solve this problem.
Step 1: Free download and open iBoysoft Data Recovery for Mac.
Step 2: Choose the unmounted external hard drive and click Search for Lost Data.
Step 3: Find and preview the search result.
Step 4: Choose a location to restore your data and click Recover.
After you recover your data, you can fix the unmounted external hard drive by using First Aid in Disk Utility to fix it and mount it on your Mac again.
If there's any problem, it will be fixed automatically; if not, you need to take another way--format the external hard drive in Disk Utility to fix it. You can take the steps as follow:
- Click Erase in the top menu in Disk Utility.
- Type in the disk name and select a format and a scheme.
- Click Erase to start.
- Click Done after finished.