Q: I'm using Azure Site Recovery to replicate virtual machines to Azure, but the virtual machine has a D drive. What happens when it fails over to Azure?
A: On a typical Azure virtual machine, the C drive contains the operating system and is stored on Azure Storage—which means it's replicated three times. A D drive is an added drive that isn't stored on Azure Storage and instead uses local storage to the host—which means the data stored on D isn't considered persistent storage, although it's useful for the pagefile and other scratch data.
If a virtual machine that already has a D frive for data is moved to Azure, such as during a failover, then by default the existing D drive is kept and the Azure temporary drive is added to the end of the drive letters (e.g., the temporary drive might be added as drive E).
The reason the disks keep their original letter is that an operating system's SAN policy is set to OnlineAll by default. If you want to make sure your operating system keeps the same drive letter, ensure that the SAN policy is set to OnlineAll. To check this value, run the following command:
diskpart Microsoft DiskPart version 6.3.9600 Copyright (C) 1999-2013 Microsoft Corporation. On computer: SAVDALWKS08 DISKPART> san SAN Policy : Online All
Microsoft documents its SAN policy on the TechNet website.