Cannot attach a new disk to DS12 VM due to "unsupported location"

Hi-

I've created a DS12-sized VM in my Azure subscription. The OS disk is currently located in a standard storage account.  The VM is attached to an existing Virtual Network.

I'm trying to create a new P10 data disk, backed by a Premium Storage account.  I've successfully created the Premium Storage account.

The problem is that when I start the "Attach a new disk" task, I am unable to choose a different storage account.  It lists my storage accounts, but it has them all disabled with a message stating that "This storage account location is not supported".

All of my resources are located in the "West US" region.

I'm using the preview Azure portal.

Any advice on how to proceed would be much appreciated.

Thanks,

Gerard