Unable to Create Empty Flexible VMSS for Existing VM Migration – Managed Disk Validation Error

Hello @Mohit Kumar (Capgemini America Inc)

Thanks for the detailed write-up. The error is expected here — let me explain why and give you the exact steps.

Why it happensaz vmss create (and the portal) build a VM scaling profile by default. On a Flexible scale set, any VM profile must reference managed disks — so when the profile is present (even partially, e.g. copied from an older Uniform/unmanaged-disk template with vhdContainers or storageUri), you get:

"...orchestration mode 'Flexible' and a Virtual Machine profile must use managed disks."

For your goal an empty Flex VMSS to attach an existing VM later you don't want a VM profile at all.

Step 1 — Create the empty Flexible VMSS (no scaling profile) Omit all image/OS/admin/SKU parameters. That's what prevents the VM profile (and the error):

az vmss create \
  --name myEmptyVmss \
  --resource-group myResourceGroup \
  --orchestration-mode Flexible \
  --platform-fault-domain-count 1

I've set --platform-fault-domain-count 1 deliberately: attaching an existing VM to a scale set with fault domain count = 1 requires no downtime. (For a brand-new empty set with no attach-without-downtime requirement, up to 3 is also valid.)

https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-scaling-profile?tabs=portal#create-a-scale-set-without-a-scaling-profile

Step 2 — Attach your existing VM

az vm update \
  --resource-group myResourceGroup \
  --name myExistingVM \
  --set virtualMachineScaleSet.id="/subscriptions/{subId}/resourceGroups/{rg}/providers/Microsoft.Compute/virtualMachineScaleSets/myEmptyVmss"

Prerequisites for the attach to succeed (these are the common silent blockers):

• Scale set is in Flexible orchestration mode with singlePlacementGroup = false
• VM and scale set are in the same resource group
• VM uses managed disks
• VM is not in an Availability Set or Proximity Placement Group
• Zonality matches — a regional (no-zone) VM attaches to a regional scale set; a zonal VM attaches only to a scale set spanning that same zone

Note: attaching a VM does not automatically wire it to any load balancer. If the VM needs to receive load-balanced traffic, configure its NIC's backend pool manually afterward.

https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-attach-detach-vm?tabs=portal-1%2Cportal-2%2Cportal-3

Links :
https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-scaling-profile?tabs=portal#create-a-scale-set-without-a-scaling-profile
https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-attach-detach-vm?tabs=portal-1%2Cportal-2%2Cportal-3
https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-orchestration-modes

Thanks,
Manish.