Detaching and reattaching disks

This document discusses how to attach and reattach boot and non-boot disks to Compute Engine bare metal and virtual machine (VM) instances.

Detaching an instance's boot disk helps you complete the following tasks without having to recreate the instance:

• Replace the boot disk for an instance with a different disk
• Repair an instance's boot disk by temporarily attaching it to another instance

Detaching a non-boot disk from one instance and attaching it to another lets you use the data on the disk from the second instance without having to copy the data to a new disk.

Before you begin

• If you haven't already, set up authentication . Authentication verifies your identity for access to Google Cloud services and APIs. To run code or samples from a local development environment, you can authenticate to Compute Engine by selecting one of the following options:

  Select the tab for how you plan to use the samples on this page:

  Console

  When you use the Google Cloud console to access Google Cloud services and APIs, you don't need to set up authentication.

  gcloud

  1. Install the Google Cloud CLI. After installation, initialize the Google Cloud CLI by running the following command:

     gcloud  
     init

     If you're using an external identity provider (IdP), you must first sign in to the gcloud CLI with your federated identity .

  2. Set a default region and zone .

  REST

  To use the REST API samples on this page in a local development environment, you use the credentials you provide to the gcloud CLI.

  Install the Google Cloud CLI.

  If you're using an external identity provider (IdP), you must first sign in to the gcloud CLI with your federated identity .

  For more information, see Authenticate for using REST in the Google Cloud authentication documentation.

Permissions required for this task

To perform this task, you must have the following permissions :

• compute.instances.detachDisk on the instance
• compute.instances.attachDisk on the instance

Move a boot disk between instances

Before you move a boot disk between instances that use different machine series, confirm the boot disk architecture of both instances match For example, you can't attach a boot disk from a C4A instance to a C4 instance because C4A and C4 use Arm and x86 architectures, respectively. To determine the architecture of a machine series, see the Architecturerow in the Machine series comparison table.

For additional factors to consider when moving a boot disk between machine series, see Move your workload to a new compute instance .

Restrictions

• The boot disk's architecture must match the architecture of the instance. For example, you can't attach a boot disk that uses x86 to an Arm instance.
• You can attach or detach a boot disk only from a stopped VM instance .
• You can attach only one boot disk per VM instance.
• If using the gcloud CLI or REST, you can attach only existing boot disks to a VM instance.
• You can't detach a Local SSD disk from an instance. You can only attach Local SSD disks to an instance when you create the instance.

Detach a boot disk

Before detaching the boot disk from the VM instance, you must stop the instance . You don't need to unmount the disk.

gcloud compute instances detach-disk INSTANCE_NAME 
\
    --disk= DISK_NAME 

POST https://compute.googleapis.com/compute/v1/projects/ PROJECT_ID 
/zones/ ZONE 
/instances/ INSTANCE_NAME 
/detachDisk?deviceName= DISK_NAME 

Reattach a boot disk

Any disk can be reattached as a boot disk to an instance as long as the instance does not already have a boot disk attached and the boot disk is in the same zone as the VM instance. If you customize a boot disk, you must ensure that the boot disk boots properly after it is attached.

Both the existing disk and the new disk must either be UEFI-compatible or not. To create a UEFI-compatible disk, use the gcloud compute disk create command with the --guest-os-features flag set to UEFI_COMPATIBLE .

gcloud compute instances attach-disk INSTANCE_NAME 
\
    --disk= DISK_NAME 
\
    --boot

POST https://compute.googleapis.com/compute/v1/projects/ PROJECT_ID 
/zones/ ZONE 
/instances/ INSTANCE_NAME 
/attachDisk
{
 "boot": true,
 "source": "zones/ ZONE 
/disks/ DISK_NAME 
"
}

After you attach the disk to your VM instance, restart the instance .

Update a boot disk for an instance

You can use only the Google Cloud console to update the boot disk for a VM in one step. Specifically, you can detach an existing boot disk and attach a new boot disk as one action.

This feature is not available through the gcloud CLI or REST.

1. Go to the VM instances page.

   Go to VM instances

2. If prompted, select your project and click Continue.
3. Click the instance for which you want to attach the boot disk.
4. If the VM is running, click stop Stopto stop the VM. If there is no Stopoption, click more_vert More actions > stop Stop.
5. Click Editat the top of the page.
6. In the Boot Disksection, click close Detach boot disk.
7. Click Add item.
8. Select a boot disk from the drop-down menu.
9. Click Save.

The Google Cloud console proceeds to detach your existing boot disk and attach the new boot disk that you specified. When this process is complete, restart the VM instance .

Attach a non-boot disk to an instance

For detailed instructions on attaching a new or existing non-boot disk to a running instance, see Attach a disk to an instance .

Detach a non-boot disk from an instance

You don't need to stop an instance to detach a non-boot disk. You can perform these operations while the instance is running.

gcloud compute instances detach-disk INSTANCE_NAME 
\ DISK_OR_DEVICE_NAME 
--disk-scope DISK_SCOPE 

POST https://compute.googleapis.com/compute/v1/projects/ PROJECT_ID 
/zones/ ZONE 
/instances/ INSTANCE_NAME 
/detachDisk?deviceName= DEVICE_NAME 

What's next?

• Create a new disk from a snapshot
• Transfer files to VM instances