Recently I was asked a question about mounting priority, in reference to External Data Recovery,specifically how the Windows 7 operating system would handle the boot sequence in an old boot drive mounted as a slave in a computer with a new boot drive. It brought to the fore front a unique query about the inner workings of Microsoft boot handlers. In order to clearly understand how Microsoft handles this, I offered the following explanation.
First, the drive mount order in the BIOS gives presentation priority to the operating system. In this case, each of the drives had been the primary boot in their respective systems so each will try and boot. What can prevent that is the order in which the drives are listed in the BIOS and which is considered the primary boot device. All BIOS offer menu selections that give you the ability to choose the boot device sequence, and the ordering of devices within the device type.
Second, once the OS boots, Windows simply prioritizes the partitions using drive order in the BIOS and then partition order using either legacy Master Boot Record, or Extensible Firmware Interface (EFI) in conjunction with Global Unique Identifier Partition Table (GPT) handlers. The partition handlers give priority and attributes to each volume as it is mounted.
Third, after boot order has been established and mountable partitions have been sorted then each volume is mounted in order. These volumes are tested for viability, index integrity, and MFT integrity under NTFS. A simple file system check is done in order to make sure that when the volume is mounted it has the integrity necessary to maintain a clean file system index and can receive data for reading and writing.
Fourth, security is then assigned to each volume. There may be some issues here that need to be addressed. Primarily, each bootable volume has a primary administrator user that has control of all file permissions. The new security on the new boot volume and the old security of the previous bootable volume parameters will be different as they are driven by a GUID. This will cause access problems for many of the files and folders on the old bootable volume. There is a method in Windows for establishing parallel security access which is outlined here.