Yes, but /dev/vda is rootfs (ephemeral store mounted on /), which is 8Gs and comes for free with every Fly VM.
Persistent store (volumes) are (from what I have observed) virtual block device /dev/vdc (ex).
So, I guess, /data hasn’t been mounted at all… I haven’t used flyctl m update with volumes, may be someone else who has will chime in. You can also choose to run LOG_LEVEL=debug flyctl m update ... to see where the cmd is going wrong.
Other than that, to unblock yourself you can try the Machines REST API: Working with the Machines API · Fly Docs