When you no longer need a data disk that's attached to a virtual machine, you can easily detach it. This removes the disk from the virtual machine, but doesn't remove it from storage. If you want to use the existing data on the disk again, you can reattach it to the same virtual machine, or another one.
A virtual machine in Azure uses different types of disks -- an operating system disk, a local temporary disk, and optional data disks. Data disks are the recommended way to store data for a virtual machine. For details, see About Disks and VHDs for Virtual Machines. It's not possible to detach an operating system disk unless you also delete the virtual machine.
If you don't know the name of the disk or want to verify it before you detach it, follow these steps.
Azure automatically assigns a name to the disk when you attach it. The name consists of the cloud service name, the virtual machine name, and a number.
If you haven't already done so, sign in to the Azure Portal.
Click Virtual Machines, click the name of virtual machine, and then click Dashboard.
Under Disks, the table lists the name and type of all attached disks. For example, this screen shows a virtual machine with one operating system (OS) disk and one data disk:
After you find the name of the disk, you're ready to detach it:
From the command bar, click Detach Disk.
Select the data disk, and then click the check mark to detach it.
The disk remains in storage but is no longer attached to a virtual machine.