Open Disk Utility (as per the steps above).

If your Mac is unable to repair the disk if is likely that the drive is either formatted using a file system that the Mac cannot read, or it is well and truly broken – if it’s the latter we suggest you follow this tutorial about If after running First Aid the Mac finds errors you could fix you may see the option to Repair Disk.Click the First Aid tab and select Run.If you need the data on the drive do not choose Erase or Restore!) Erase deletes all the data stored on the drive. (Restore allows you to erase the contents of the drive and replace that with data from somewhere else. First Aid will check the disk for errors and then repair the disk if necessary and this is the option to choose. Your options are First Aid, Erase and Restore.(If there is no volume listed your Mac is not able to access the drive. If your Mac has already mounted the drive the option Unmount will be displayed instead. If it is there click on it and select Mount. If you can see the hard drive in Disk Utility check underneath it for a volume.Look in the column on the left to see if the hard drive appears there.Find Disk Utility by opening Spotlight (cmd+Space-bar) and start typing Disk Utility, press enter to open the program.If the various checks above suggest that the disk is faulty then you can use Apple’sĭisk Utility program to access the disk and potentially fix whatever is causing the issue. If the drive mounts on the PC it’s likely that you have discovered what the problem is: the drive is formatted for PCs and can’t be read by your Mac. The next step is to try plugging the drive into a PC. If it also fails to mount there you will know that there is a problem with the drive while if it does mount then the problem is with your Mac. The next step is to try plugging the drive into another Mac.

How to access usb drive on mac terminal