Hello Azure Scriptum
After you have attached disk to VM in Azure Portal, you need to initialize a new data disk at OS level
- Connect to the VM.
- Select the Windows Start menu inside the running VM and enter diskmgmt.msc in the search box. The Disk Management console opens.
- Disk Management recognizes that you have a new, uninitialized disk and the Initialize Disk window appears.
- Verify the new disk is selected and then select OK to initialize it.
- The new disk appears as unallocated. Right-click anywhere on the disk and select New simple volume. The New Simple Volume Wizard window opens.
- Proceed through the wizard, keeping all of the defaults, and when you're done select Finish.
- Close Disk Management.
- A pop-up window appears notifying you that you need to format the new disk before you can use it. Select Format disk.
- In the Format new disk window, check the settings, and then select Start.
- A warning appears notifying you that formatting the disks erases all of the data. Select OK.
- When the formatting is complete, select OK.
https://learn.microsoft.com/en-us/azure/virtual-machines/windows/attach-managed-disk-portal