The easiest method would be to make a symbolic link from the default
/orchives directory to your USB hard drive.
First find where your USB hard drive is mounted:
Let's assume it's at /media/pi/hdd (yours will be different). Now you can stop Orchid, and move the contents of your existing
/orchives directory to the hard drive.
sudo systemctl stop orchid
sudo mkdir /media/pi/hdd/orchives
sudo mv /orchives/* /media/pi/hdd/orchives
sudo rmdir /orchives
Now make /orchives a symbolic link to the directory on your hard drive, and restart Orchid:
sudo ln -s /media/pi/hdd/orchives /orchives
sudo systemctl start orchid