Azure Powershell Save-AzureVhd fails with vague error: One or more errors occurred

I am trying to download a Azure vhd to local disk. The process starts, creates the file, reports progress then exits with the below message.

Save-AzureVhd -Source $sourceVHD -LocalFilePath $destinationVHD -NumberOfThreads 5 -verbose
Elapsed time for download: 00:00:37
Save-AzureVhd : One or more errors occurred.
At line:4 char:1+ Save-AzureVhd -Source $sourceVHD -LocalFilePath $destinationVHD -NumberOfThreads ...+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+ CategoryInfo          : NotSpecified: (:) [Save-AzureVhd], AggregateException+ FullyQualifiedErrorId : System.AggregateException,Microsoft.WindowsAzure.Commands.ServiceManagement.StorageServices.SaveAzureVhdCommand