My hard disk won’t mount
Make sure the disk is correctly connected, powered up and switched on. Try disconnecting and reconnecting it. If that doesn’t work, launch Disk Utility. If the disk is displayed on the left panel of Disk Utility’s main window, click Mount in the toolbar along the top.
If it doesn’t mount, click First Aid and then click Repair to fix the disk. If Disk Utility can’t repair it, you’ll see a dialog box telling you it was unable to help. Use a tool like DiskWarrior (alsoft.com) to try and repair it. If it’s not there, your Mac can’t see it. Try it on another computer. You may have to remove the disk from the chassis and try it with another case.
Don’t let the red octagon of doom strike the fear into you; a fix is available.