To automatically mount an external USB drive with an EXT4 partition at system startup using /etc/fstab, follow these steps: Identify the Partition.

Determine the UUID (Universally Unique Identifier) or the LABEL of your EXT4 partition on the USB drive. The UUID is generally preferred for its uniqueness and stability, especially if the device name (/dev/sdXn) might change.

    sudo blkid

This command will list all block devices and their UUIDs and labels. Locate the entry corresponding to your USB drive's EXT4 partition. For example, it might look like:

    /dev/sda1: UUID="your_uuid_here" TYPE="ext4" LABEL="your_label_here"

Create a Mount Point.
Create a directory where the USB drive will be mounted. This directory will act as the access point for the partition's contents. Choose a logical and accessible location, for example, within /mnt or /media.

    sudo mkdir -p /mnt/usb_ext4

Edit /etc/fstab.
Open the /etc/fstab file with a text editor with root privileges.

    sudo nano /etc/fstab

Add a new line to the end of the file following this format:

    UUID=your_uuid_here /mnt/usb_ext4 ext4 defaults,nofail 0 2

    UUID=your_uuid_here: Replace your_uuid_here with the actual UUID you identified in step 1. You can also use LABEL=your_label_here if you prefer to use the label.

/mnt/usb_ext4: Replace with the mount point you created in step 2.
ext4: Specifies the filesystem type.
defaults,nofail: These are mount options.

    defaults: Includes common options like rw (read/write), suid, dev, exec, auto, nouser, and async.

nofail: This is crucial for USB drives. It prevents the system from halting during boot if the USB drive is not connected.

0: The dump frequency (usually 0 for no dumping).
2: The pass number for fsck (filesystem check) at boot. A value of 2 means fsck will be performed after the root filesystem.

Save and Exit.
Save the changes to /etc/fstab and exit the text editor. Test the Mount.
To verify the entry without rebooting, run:

    sudo mount -a

If there are no errors, the USB drive should now be mounted at your specified mount point. You can check its status with:

    df -h /mnt/usb_ext4

Set Permissions (Optional but Recommended).
If you need specific user permissions for the mounted drive, adjust the ownership and permissions of the mount point after mounting, or include uid and gid options in the fstab entry if you want to set specific user/group ownership at mount time (though defaults often suffices for basic read/write access for the user who owns the mount point).

    sudo chown -R your_username:your_group /mnt/usb_ext4

Replace your_username and your_group with your actual user and group.
After these steps, your EXT4 partition on the USB drive will be automatically mounted at the specified location each time your system boots, as long as the drive is connected.