Do you initialized and partitioned your 2 hard drives? I see that you have 3 partitions on SSD, but I don’t see any on other hard drives. If you didn’t, you can follow DigitalOcean tutorial - How To Partition and Format Storage Devices in Linux.

Once you do this, or if you are already done, you can proceed.

/var/ and all other directories and files there (/var/www/html and so on) are on root / file system. Your root file system is on your SSD. To overcome this, you have two possibilities. You can change your nginx server block to use another location for /var/www/html. Location of files are defined by root directive. Example server block:

server {
        listen 80 default_server;
        listen [::]:80 default_server;

        root /var/www/html;
        index index.html index.htm index.nginx-debian.html;

        server_name _;

        location / {
                try_files $uri $uri/ =404;
        }
}

Root directive is what you want to change. After change is:

server {
        listen 80 default_server;
        listen [::]:80 default_server;

        root /mnt/www/html;
        index index.html index.htm index.nginx-debian.html;

        server_name _;

        location / {
                try_files $uri $uri/ =404;
        }
}

This is most easiest solution. Change /mnt/www/html with location where you mounted hard drive. There you also need to upload files.

Second and riskier solution is to try to mount your hard drive to /var/www. In that case, you can go with default nginx config, but you should change /mnt/data from above linked tutorial to /var/www. Please be very careful when doing so! There is risk behind this, I didn’t tested this myself, but AFAIK it should work in theory!