Failed to start VM after resizing disk from 128GB to 256GB

Brandon Lee 20 Reputation points
2026-06-29T09:04:54.0566667+00:00

Hello Azure Support Team,

I am currently experiencing an issue where my virtual machine fails to start. Here are the exact steps I took before encountering this problem:

I stopped (deallocated) the server instance.

I increased the disk size from 128GB to 256GB.

I attempted to start the server again, but the operation failed.

Could you please guide me on how to resolve this issue and get the server running again?Screenshot 2026-06-29 at 4.04.10 PM

Thank you for your time and assistance.

Azure Virtual Machines
Azure Virtual Machines

An Azure service that is used to provision Windows and Linux virtual machines.

Answer recommended by moderator
TP 163.6K Reputation points Volunteer Moderator
2026-06-29T09:33:06.2333333+00:00

Hi Brandon,

Based on the error you received there was not enough physical host capacity in the Azure availability zone to allocate to your VM so that it could start. This has nothing to do with you resizing the OS disk.

What I recommend is to periodically try to Start the VM over the next several hours until it succeeds. If you keep trying without success and can no longer wait for more capacity to free up, what you can do is try resizing to different size family, or re-create the VM in different availability zone. For example, say the current VM size uses Intel CPU, try resizing to equivalent size using AMD instead.

Please click Accept Answer and upvote if the above was helpful.

Thanks.

-TP

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  1. Alex Burlachenko 25,290 Reputation points MVP Volunteer Moderator
    2026-06-29T09:28:44.8766667+00:00

    Brandon Lee hi & thx for sharing urs issue here at Q&A portal,

    Disk resize itself usually shouldn’t stop a VM from starting.

    From the screenshot, the OS disk provisioning succeeded, but the guest agent is Unknown. So the problem is likely boot/guest OS side, not the disk resize operation.

    Check Boot Diagnostics and Activity Log for the failed Start VM operation. That should show the real error.

    https://learn.microsoft.com/en-us/azure/virtual-machines/boot-diagnostics

    https://learn.microsoft.com/en-us/azure/virtual-machines/troubleshooting/serial-console-overview

    If it still won’t boot, use a repair VM and attach the OS disk there to inspect logs/filesystem. Don’t delete the VM or disk.

    rgds,

    Alex

    &

    If my answer was helpful pls mark it and additional thx if u follow me at Q&A portal

    and at my blog https://ctrlaltdel.blog/

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  2. AI answer

    2026-06-29T09:05:21.89+00:00

    The resize steps taken (stop/deallocate VM, then increase disk size) are correct and supported for OS disks. The failure to start is likely due to a provisioning or boot issue that occurred after the resize.

    Use the following checks and recovery steps:

    1. Confirm the disk resize completed successfully
      • In the Azure portal, open the disk resource attached to the VM.
      • On Size + performance, verify the new size (256 GB) is shown and the last resize operation is Succeeded.
      • If the resize operation shows Failed, review the error in Notifications or the disk Activity log to identify the specific error code and message.
        • In the portal, select the bell icon (Notifications), open the failed resize operation, and review the details.
        • Or go to the disk → Activity log, filter for Failed operations, and open the failed operation. The JSON tab shows the error code and message.
    2. Ensure the VM is fully deallocated before retrying any disk changes
      Azure distinguishes between Stopped and Stopped (deallocated): shutting down from inside the OS leaves the VM in Stopped, which still holds leases. For OS disks, online resize is not supported.
      • In the portal, go to the VM and verify the status is Stopped (deallocated). If it is only Stopped, select Stop in the portal and wait until it shows Stopped (deallocated).
    3. If needed, reapply the VM state to fix provisioning errors
      If the VM is stuck in a failed provisioning state after the disk resize/start attempt, reapplying the VM can clear the failure:
      • In the Azure portal, open the VM that failed to start.
      • Under Help, select Redeploy + reapply.
      • Choose Reapply.
        This reruns VM provisioning and can resolve failures that occurred during a previous action.
    4. Retry starting the VM
      After confirming the disk resize succeeded and reapplying the VM state (if needed):
      • Go back to the VM and select Start.
      • If the start operation fails again, immediately check Notifications and the VM Activity log for the new error code and message, as in step 1.
    5. If the VM boots but the OS still shows 128 GB
      Once the VM starts successfully, the guest OS may still report the old disk size. In that case, extend the partition and filesystem from inside the OS to use the new 256 GB. The Azure portal showing 256 GB while the OS shows 128 GB is expected until the OS-level resize is completed.

    If the VM continues to fail to start even after reapplying the VM state and confirming the disk resize operation succeeded, collect the error code from the failed start operation in the Activity log and use it to drive further troubleshooting or share it with support.

    References:

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