Using an extra disk on a Windows VM

A newly attached disk appears in Windows as an empty disk without a drive letter. Windows may also mark it Offline. These steps bring it online and make it a new drive, such as D:.

With Disk Management

1. Connect to the VM with Remote Desktop and sign in as an administrator.
2. Right-click the Start button and choose Disk Management (or run diskmgmt.msc).
3. If Windows asks to initialize the new disk, choose GPT and click OK, then go to step 6.
4. Otherwise, find the new disk in the lower half of the window: it has the size of the disk you requested and shows Unallocated. If it says Offline, right-click the disk's label on the left (for example Disk 1) and choose Online.
5. Right-click the label again and choose Initialize Disk, select GPT, and click OK.
6. Right-click the Unallocated area and choose New Simple Volume.
7. In the wizard, accept the maximum size, choose a drive letter, keep NTFS, give the volume a name such as Data, and finish.

The new drive appears in File Explorer a few seconds later.

With PowerShell

The same steps in one command, from PowerShell run as administrator. It only touches disks that are still empty (RAW), so existing disks are not affected:

Get-Disk | Where-Object PartitionStyle -eq 'RAW' |
  Set-Disk -IsOffline $false -PassThru |
  Initialize-Disk -PartitionStyle GPT -PassThru |
  New-Partition -AssignDriveLetter -UseMaximumSize |
  Format-Volume -FileSystem NTFS -NewFileSystemLabel 'Data' -Confirm:$false

Growing the C: drive instead

Windows cannot extend C: onto a second disk. Keep data on the new drive; if you need a larger system drive, write to support.