VM with RDM Disk Fails to Power On with “Unable to enumerate all disks” Error

Products

• VMware vSphere ESXi

Issue/Introduction

• A virtual machine (VM) fails to power on after being migrated (vMotion) to a new ESXi host.
• The power-on task fails with the error message:  “Unable to enumerate all disks.”
  
  

• In the vSphere Client:
• Select the affected VM.
• Click Edit Settings.
• Multiple VMDKs are listed, but one or more VMDKs show a disk size of 0 MB.
• The VM powers on successfully when vMotion back to the original ESXi host.

Environment

VMware vSphere ESXi 8

Cause

This issue occurs when the backend storage device for one or more Raw Device Mapping (RDM) disks is not visible to the target ESXi host.

Although the VM configuration still contains the RDM mappings,
the underlying storage LUN backing the RDM disk is detached or not presented to the destination ESXi host.
As a result, ESXi is unable to enumerate all disks during VM power-on, causing the operation to fail.

Resolution

Ensure that all back end storage devices for the RDM disks are properly attached and visible to the ESXi host on which the VM is running.

Steps:

1. Identify all RDM disks attached to the VM.
2. Verify the back end storage devices (LUNs) corresponding to each RDM disk.
3. On the target ESXi host:
• Confirm that the RDM back end storage devices are presented from the storage array.
• Verify storage connectivity and device visibility.
4. If any RDM back end storage device is missing:
• Attach or re-present the storage device to the ESXi host.
5. Rescan storage adapters on the ESXi host.
6. Power on the virtual machine.

After the RDM back end storage devices are correctly attached, the VM should power on successfully without the “Unable to enumerate disks” error.