Properly unmounting and ejecting the external hard drive on Mac could prevent damaging the drive data. If you can't unmount or eject the drive from Mac and see the disk in use error, it indicates some processes are accessing the drive. Therefore, you can quit these processes and try to unmount the drive again.
You may encounter these error messages when you fail to unmount and eject the external hard drive:
To fix unable to unmount the external hard drive on Mac, you can first quit apps and processes that are reading or writing data to the drive. Use Force Quit to close apps on Mac, or open Activity Monitor and check Disk Activity from the menu bar View > Dock Icon > Show Disk Utility, then double-click and quit the suspicious ones that hinder the system from unmounting the drive.
Alternatively, restarting Mac is a quit fix that straightforwardly closes all running processes and starts from scratch, this also terminates the processes disk engaged. After restarting, you may unmount and eject the external hard drive without a problem. Remember to save your working projects before restating to avoid data loss.
If these methods don't work, the external hard drive may be corrupted, you can repair the drive with Disk Utility's First Aid. If there is any data loss on the drive, try iBoysoft Data Recovery for Mac to get them back ASAP!