Attaching a disk when Mac cannot be started

If the Mac cannot be started, mount the disk to solve the problem [*] connecting the mobile hard disk, and then starting the Mac to single-user mode (when starting, press and hold the Java code command + s). Remember, you must connect to the mobile hard drive before starting it. Plug it in halfway. I have tried it several times and I have not succeeded. Follow the prompts and enter the Java code/sbin/mount-uw/and Java code/sbin/fsck-fy to mount the hard disk and check the disk in read/write mode. At this time, if your situation is the same as mine, the Java code/sbin/fsck-fy should report an error at this time, but it does not matter, and then proceed to the next operation. Execute the Java code ls/Volumes command to display all mounted disks. If your mobile hard disk or USB flash disk is not displayed here, it means it has not been mounted yet. It does not matter. If it is not mounted, we will first create a mount point here. For example: Java code mkdir/Volumes/usb we need to determine the disk Number of the mounted Mobile storage device: Java code ls/dev/disk * in general, the mobile storage device number is at the end, for example,/dev/disk1s1. Now, mount the Mobile storage device to the mount point just created and run the following command: java code/sbin/mount_msdos/dev/disk1s1/Volumes/usb because my hard disk format is FAT, if the format of your hard disk or usb disk is not this, you can find the mount tool in the/sbin/directory and execute the command. If the mounting is successful, you can run the Java code ls/Volumes/usb to view the files in the mobile storage device. You can try to copy files from the hard drive on the machine to the mobile storage device: Java code cp/Users/your_user_name/Documents/somefile/Volumes/usb/if successful, then you can back up your important data. Backup: [*] Java code cp/Users/your_user_name/Documents/somefile/Volumes/usb/if successful, you can back up your important data.