Virtual Machine: Unable to Detach Locked Data Disk and Delete in Failed State

Hello Nelsen Lim

Thank you for contacting Microsoft Azure Technical Support. I have gone through the details you have provided about the issue you are facing.

Here is the action plan we need to follow:
Based on the symptoms, the resource lock is the blocker, not the VM's Failed state. Azure cannot detach the managed disk while a CanNotDelete or ReadOnly lock exists on the disk (or inherited from the resource group/subscription).

Recommended approach

Step 1: Identify where the lock is applied

Check whether the lock exists on:

The managed disk itself

The resource group (virtualmachine-rg)

The subscription

Remember that some locks applied at a higher scope are inherited by child resources.

Azure CLI:

 az lock list --resource-group virtualmachine-rg -o table

 az lock list --resource "/subscriptions/<sub-id>/resourceGroups/virtualmachine-rg/providers/Microsoft.Compute/disks/<disk-name>" -o table

Step 2: Temporarily remove the lock

If the disk must be preserved, it is safe to remove the lock temporarily, detach the disk, and reapply the lock afterward.

Portal:

Navigate to the managed disk RG.

Select Locks.

Delete the lock.

Or CLI: 

az lock delete --name <lock-name> \

--resource-group virtualmachine-rg \

--resource-name <disk-name> \

--resource-type Microsoft.Compute/disks

Step 3: Detach the data disk

If the VM resource is still manageable:

 az vm disk detach \

--resource-group virtualmachine-rg \

--vm-name <vm-name> \

--name <disk-name>

If the portal allows editing the VM, remove the data disk from the Disks blade and save.

Step 4: Delete the VM while retaining disks

Delete the VM resource only: 

az vm delete \

--resource-group virtualmachine-rg \

--name <vm-name> \

--yes 

Alternatively, if the VM is stuck in a Failed state and normal deletion hangs, try:

 az vm delete \

--resource-group virtualmachine-rg \

--name <vm-name> \

--force-deletion true \

--yes

Step 5: Reapply the lock to the disk

Note: Ensure, you select the specific VM resources that you want to delete along with VM, except disk.

After confirming the VM is deleted and the disk remains:

az lock create \

--name PreserveDisk \

--lock-type CanNotDelete \

--resource-group virtualmachine-rg \

--resource-name <disk-name> \

--resource-type Microsoft.Compute/disks

If detachment still fails

If the disk remains in an attached or toBeDetached state even after removing the lock, there may be a backend disk-management issue, and we will have to investigate further Reference Official Document Link: az resource lock | Microsoft LearnI have gone through the details you have provided about the issue you are facing, and I see the AI support tried to help you, while you have requested a human support over team's session. I am happy to connect with you to troubleshoot this issue. Let me know what time works best for you. 

Reference: az resource lock | Microsoft Learn