Hi there,

What I would do is first mount the disk to a temporary directory, then by using the rsync or the cp command I would copy the files from the current directory to the volume.

Once the files are in place, I would remove the existing directory from the root partition, and then I would mount a volume to that specific directory path instead.

You could run the following command:

sudo mount -o defaults,nofail,discard,noatime /dev/disk/by-id/scsi-example /var/www/your_site/example_mount_point

This is from the documentation here:

https://docs.digitalocean.com/products/volumes/how-to/mount/

Let me know if you have any questions!
Regards,
Bobby