By default, Azure virtual machines are assigned to drive D for use as temporary storage. This drive assignment causes all other attached storage drive assignments to increment by one letter.
For example, if your on-premises installation uses a data disk that is assigned to drive D for application installations, the assignment for this drive increments to drive E after you import the VM to Azure. To prevent this automatic assignment, you can set the storage area network (SAN) policy to "OnlineAll." This setting causes Azure to assign the next free drive letter to its temporary volume. In this situation, the data drive maintains the drive D designation, and the Azure temporary drive is assigned drive E.
To view the current SAN policy from the guest system, follow these steps:
- On the VM (not on the host server), open an elevated Command Prompt window.
- Type diskpart.
- Type SAN.
If the drive letter of the guest operating system is not maintained, this command returns either "Offline All" or "Offline Shared."
To make sure that all disks are brought online and are both readable and writeable, set the SAN policy to OnlineAll. To do this, run the following command at the DISKPART prompt:
Azure Support Team Blog article.