Windows Server and configured OS on a 400 GB disk. (RAID 1 on embedded PERC H755).  Disk manager lists - Disk 0 - [EFI Partition, OS Partition, Recovery Partition]. I expected this.  Added second disk, 800GB (same RAID 1 on embedded PERC H755) Disk manager lists the new disk as - Disk 0 and changes the OS drive to Disk 1 - [EFI Partition, OS Partition, Recovery Partition].  Not what I expected.  Added third disk, 17 TB (RAID 1 on PCIe PERC H755) and Disk numbers all shift again. Disk ) = 17TB, Disk 1 = 800 GB, Disk 2 400GB - [EFI Partition, OS Partition, Recovery Partition].  Change of drive positions and controller made no difference.  BIOS or UEFI made no difference.  Any clue why it is shifting disk #?