Explanation
You can find unused disks in the Azure Storage Explorer console.Once you drill down to the Blob containers under a storage account, you can see the lease state of the residing VHD (the lease state determines if the VHD is being used by any resource) and the VM to which it is leased out. If you find that the lease state and the VM fields are blank, it means that the VHD in question is unused. The screenshot below shows two active VHDs being used by VMs as data and OS disks. The name of the VM and lease state are shown in the "VM Name" and "Lease State" columns, respectively.

Reference:
https://cloud.netapp.com/blog/reduce-azure-storage-costs