They no longer match the disk signature of a drive and therefore drive letter assignments are lost. If a disk signature is changed the partition numbers in the registry become invalid. During Windows bootup the stored partition number is compared with the disk signature and if they match then the partition will retain the drive letter that was previously assigned to it. On a classic MBR styled hard drive this unique partition number is generated from the disk signature of the drive and the sector number that a partition starts on, ( It's different on GPT drives - see box below).
With NT operating systems the hard drive partition drive letters are maintained by linking them to a partition by using a unique number that is assigned to each partition and stored in the Windows registry. This differs from the Win9x generation of operating systems where drive letters were assigned on every bootup in a default order as the partitions and drives were discovered, which meant that a change to partitions or the adding or removal of drives would cause drive letters to change. The Windows NT operating system from before NT4 till Win-8 will remember the drive letter that has previously been allocated to a partition or drive and always reassign that same letter every time the operating system is booted. They are linked to a hard drive's Disk Signature.
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