Unable to modify Azure VMSS reference image to an image without data disks

Question

My Azure VMSS is deployed successfully and operating as expected. It uses an Azure Compute Gallery reference image (which includes both an OS disk, and 1x Data Disk).

We are now changing all our Compute Gallery reference images to single OS disk-only (since the Terraform Windows VM resource does not support deploying from an image version that has a data disk included, even though Terraform Windows VMSS resource does support it), but when changing the VMSS from dual-disk image, to the single-disk image, Terraform fails with:

Code="PropertyChangeNotAllowed" Message="Changing property 'dataDisks' is not allowed." Target="dataDisks" 

NOTE: I have successfully tested the change from the other way around: ie. Updating a VMSS that runs from a single disk image, to pointing to an image that uses 2 disks. This completed fine. It only fails when trying to go from 2 disks down to 1 disk.

(Using: Terraform v1.0.0; AzureRm Provider v3.0.2)

Answer 1

SOLUTION/WORKAROUND:

So, doing it via Terraform fails, however, with a little help from the portal, we can achieve the desired result. In the portal, on the VMSS, under Settings, select Disks. Detach the data disk from the VMSS. Takes a few seconds. Once that is detached, now run the Terraform again to update the image reference to an image with single disk. This time it completes successfully.

Hope this helps someone out there