KB937251: The disk drive numbers may not correspond as expected to the SATA channel numbers when you set up Windows Vista, Windows Server 2008, Windows Server 2008 R2, or Windows 7 on a computer that has multiple SATA or RAID disks
In addition to notify readers that the disk numbers and SATA channels numbers may not match, this KB took a step further to explain the creation of 100MB system partition.
This is its original content:
Additionally, when you install Windows 7, the installation creates a 100MB system partition alongside the Windows partition by default. The system partition is created to accommodate a Bitlocker requirement. Bitlocker requires that the boot files and Windows files are located on separate partitions. By default, the 100MB system partition is created on the disk that is detected as Disk 0. If you install Windows to a disk other than Disk 0, the 100MB system partition will still be created on Disk 0 during the installation.
Content to clarify:
The 100MB system partition will only be created on Disk 0 during the installation, if the user didn't change any of the pre-selected default settings. This by default definition includes: language selection, what type of installation do you want, which disk to install the Windows operating Sytem to...etc. If the user changes any of the pre-selected default settings, then this 100MB system partition is not guarantedd to be installed on Disk 0 as described in this KB.
I have a customer that chose "adanved options" and selected Disk 1 to install the Windows Operating System. Then he found out that the 100MB system partition is created on Disk 1 instead of Disk 0. This is because when he chose "advanced options" to make a selection change, this is no longer considered "by default", hence, the 100MB system partition is not guarantedd to be installed on Disk 0 as result.