Adding Storage to Windows Storage Pool

Some time ago, I encountered an interesting issue while attempting to expand a Storage Pool for a Windows Server running Azure Backup Server/DPM. I had no luck finding a solution at the time, so I wanted to be sure and document my recent findings here.
Say you’ve configured a Windows Storage Pool and you just added additional storage to the server to expand it. For example, consider a Storage Pool configured for use with Microsoft Azure Backup Server (which leverages System Center DPM). You navigate to the Storage Pool configuration on your server (Server Manager > File and Storage Services > Storage Pools) and add the new storage (found in the Primordial pool),

add the Physical Disks to the Storage Pool…

…and all done! Storage Pool expanded.

However, your disk volume is still at its previous capacity.

To expand this volume, you need to update its Virtual Disk. This can be found under Storage Pools in Server Manager.

Right-click the disk and Extend Virtual Disk. Set to Max size and click OK…

…and that will extend your…wait! What’s this?

An error appears: “Error extending virtual disk: Not enough available capacity.” You double-check your Storage Pool and find there’s enough capacity. So, what’s going on?
This is one of those oddball issues that’s not specifically documented. It turns out that sometimes, Windows Server is unable to automatically identify new, uninitialized storage. This storage then appears as Unspecified, which cannot be used for a virtual disk. To find the issue, run the following PowerShell command:

Get-StoragePool "DPM Storage Pool" | Get-PhysicalDisk | FT FriendlyName,Size,MediaType,HealthStatus,OperationalStatus -AutoSize

Note the Media Type set for two of the disks. Since Windows Server cannot use Unspecified disks, we’ll need to update these to HDD or SSD (whichever is appropriate). To do so, run the following PowerShell command:

Get-StoragePool "DPM Storage Pool" | Get-PhysicalDisk | Where MediaType -eq "Unspecified" | Set-PhysicalDisk -MediaType HDD

Then recheck the disks’ Media Type. You’ll see them updated as HDD.

Now we can go to our Virtual Disk in Server Manager and extend it.

And expand the volume in Disk Management as you would normally.

And voilà! There you go!
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