Update download-vhd.md

Added line to SAS URL note re VM start error and resolution.

Author: SCuffy

articles/virtual-machines/windows/download-vhd.md

@@ -95,7 +95,9 @@ az disk grant-access --duration-in-seconds 86400 --access-level Read --name your
 
 
 > [!NOTE]
-> The expiration time is increased from the default to provide enough time to download the large VHD file for a Windows Server operating system. Large VHDs can take up to several hours to download depending on your connection and the size of the VM. 
+> The expiration time is increased from the default to provide enough time to download the large VHD file for a Windows Server operating system. Large VHDs can take up to several hours to download depending on your connection and the size of the VM.
+>
+> While the SAS URL is active, attempting to start the VM will result in the error **There is an active shared access signature outstanding for disk** *diskname*. You can revoke the SAS URL by selecting **Cancel export** on the **Disk Export** page.  
 
 ## Download VHD