How to find a boot disk from an instance.

<pre lang="bash">
gcloud compute instances describe servername \
--format='get(disks[0].source)' \
--zone=us-central1-c \
--project project-id

Result

<pre lang="bash">
https://www.googleapis.com/compute/v1/projects/project-id/zones/us-central1-f/disks/servername-boot