external hard drive wont mount

Hello chama9714,

Welcome to Apple Support Communities. 

We understand that your external hard drive doesn’t always mount when you restart your Mac. We are happy to help in any way that we can. We are recommending some isolating troubleshooting, which will help us to isolate the cause.

Please use the steps as listed here to start in safe mode: How to use safe mode on your Mac - Apple Support

Intel processor

1. Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.
2. Release the key when you see the login window, then log in to your Mac. 
3. You might be asked to log in again. On either the first or second login window, you should see ”Safe Boot” in the upper-right corner of the window.

We also recommend that you test in a new user account using the steps as listed here: Change Users & Groups settings on Mac - Apple Support

Create a new user or group.

Click the New Account pop-up menu (you may be asked to enter your password), then choose a type of account.

• Administrator: An administrator can add and manage other users, install apps, and change settings. The new user you create when you first set up your Mac is an administrator. Your Mac can have multiple administrators. You can create new ones, and convert standard users to administrators. Don’t set up automatic login for an administrator. If you do, someone could simply restart your Mac and gain access with administrator privileges. To keep your Mac secure, don’t share administrator names and passwords.
• Standard: Standard users are set up by an administrator. Standard users can install apps and change their own settings, but can’t add other users or change other users’ settings.
• Sharing Only: Sharing-only users can access shared files remotely, but can’t log in to the computer or change settings. To give the user permission to access your shared files or screen, you may need to change options in File Sharing, Screen Sharing, or Remote Management settings. See Set up file sharing and Share the screen of another Mac.
• Group: Groups allow multiple users to have the same access privileges. You control a group’s access to your shared folders by setting access permissions in the File Sharing options in Sharing settings.

Let us know if you have further questions.

Take care.

That's great news, chama9714. We look forward to hearing what steps you took to resolve the issue.

Have a good one!

Would love to hear how you solved this issue - I'm trying to back up my HD and the WD drive won't mount. Disk utility says the drive is "read only" (it's my Time Machine drive).

Thanks!