In Azure, every VM has a persistent OS Disk that is used for booting the VM and contains the operating system. In addition to an OS disk, each VM also has a temporary disk that is present on the compute cluster hosting your VM.
Why is my OS disk not @ /dev/sda?
In most cases the OS disk is /dev/sda but in some odd cases it is not. This is not an Azure specific issue but a default Linux behavior. The ordering in which Linux enumerates SCSI devices is nondeterministic. It scans SCSI devices in the order they appear on the PCI Bus. Therefore, whichever disk is enumerated first is /dev/sda.
