Hi @Rudolf Tse (DWS)
Welcome to the Microsoft Q&A Platform! Thank you for asking your question here.
Make sure the VM is set up to send disk space data to Azure Monitor. You can check this by going to the Azure portal, selecting your VM, and looking under "Insights" in the "Performance" section to see if disk space metrics are available.
Double-check the time period you’ve selected. If the VM was recently set up, it might not have enough data yet, so try adjusting the time range accordingly.
The pre-built query may have some strict filters or settings that are limiting the results. You can open the query in the editor, remove unnecessary filters, and see if that helps bring up the data.
Since your custom query works, you can continue using that. You can even save it for future use if it consistently gives you the right results.
If you have any further queries, do let us know.
If the answer is helpful, please click "Accept Answer" and "Upvote it".