How XP assigns drive letters

Source

For a drive which XP has no drive letter assignment stored it assigns the first available drive letter. The search goes upwards in the alphabet and starts at 'A' for floppy drives, at 'D' for CD-ROM drives and at 'C' for all other types.

XP can save exactly one assignment per drive letter. So when a drive is attached again it gets the same letter as before. But the drive letter is not reserved while a drive is not present and therefore reassigned to another drive if required. When this happens the former assignment is overwritten and the first drive gets the first available drive letter again even its former drive letter is available when it's being attached. The former assignment just doesn't exist anymore.

Network and substs drives are completely unconsidered by XP here. XP has no problem assigning a drive to letter which is currently used as network share! Microsoft knows that this is 'a problem' and since December 2007 a hotfix is available (WindowsXP-KB297694-x86-ENU.exe). The Service Pack 3 will fix it too.

To get persistent drive letters for external drives assign one exclusive letter per drive.

My USB Drive Letter Manager solves all this.