Complete Terminal Command To Mount HD

Boot into Safe Mode according to Start up your Mac in safe mode - Apple Support and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to nearly twice as long as a normal boot as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Clique of One wrote:

I want to mount, NOT TIME MACHINE, using Terminal.

What is the correct command?

And if successfully mounted, what is the best way to keep it from ejecting?

Of all the files that are on it, there are only three left to transfer over to another hard drive, but it keeps ejecting. 

From Terminal you can access the man page, copy & Paste:

 man mount | more

for example, copy & paste:

diskutil mount 

Notice the trailing blank space, you can drag & drop the drive from the desktop to the terminal window to complete the path.

ref: Mount & Unmount Drives from the Command Line in Mac OS ...

What is the exact issue here? Having to use Terminal to mount would indicate a problem of some sort...

A drive that keeps ejecting is in a state of failure, and beyond your control.

Your life should never be reliant on a single backup when it it decides to fail—simply replace it.

3-2-1 Backup Strategy: three copies of your data, two different methods, and one offsite.