Found the UUID of your USB drive by:
```
sudo blkid
```
Add a new line in `/etc/fstab` like:
```
UUID=93ddc2e0-48e1-457c-9d59-f6a34596e6e1 /media/SSD-120GB ext4 defaults,nofail 0 0
```
- Use UUID rather than the device node (/dev/sda1) as the device node can change across reboots especially if drives are added or removed.
- Include "nofail" in the mount options to prevent the Pi hanging if it is booted without the drive attached.
Save and close /etc/fstab
Connect the HDD and reboot