Without knowing DigitalOcean’s platform exactly, I’ll try to answer.
Block Storage volumes are not located on the same disk. It is probably something like ZFS, spread over multiple disks, clustered in a SAN-like-device.
So several volumes might be on the same physical device, just like several droplets might be on the same physical hypervisor.
Currently it possible to find droplet neighbors, but something similar does not exists for volumes. Most likely because there is only one (or a few) SANs controlling all the volumes.
I would not worry about having your data in a single volume, but always make sure to have backups in multiple locations.
You biggest worry should probably be in case of data center (or SAN) failure - would you be okay with downtime for a day? Otherwise you should probably consider replicating your data over multiple data centers and you could either set it up as hot failover or have it running in a high availability setup.