Download USB Drive Letter Manager, to force Windows keep permanent drive letters
USB Drive Letter Manager is a free (for personal use) utility that will let you assign permanently a letter to a usb drive, a feature Windows Disk Management won’t provide.
USB Drive Letter Manager does not have a graphical user interface and is not that easy to use. You can configure it only by editing its .ini file
Running as service USB Drive Letter Manager automatically solves conflicts between USB drives and network or subst drives, moreover, it keeps default / permanent letters for USB drives.
Features: check if the letter is used by a network share of the currently logged on user and assign to a USB drive the next letter that is really available * reserve letters, so they are not used for local drives * assign a letter from a list of new default letters, also dependent on many different criteria as the active user, drive type, connection (USB, FireWire), USB port, volume label, size and others * assign letters for a specific USB drive by putting an INI file on the drive * remove the drive letters of card readers until a card is inserted * show a balloon tip with the assigned drive letter(s) * define autorun events depending on several criteria, and more.
All functions are applied to USB drives at the moment they are being attached, when the USBDLM service starts up and when a user logs on. USBDLM runs as Windows service.
Here is brief information I hope you find useful, explaining what you should do in order to prevent Windows from changing a letter you don’t want changed. USB Drive Letter Manager can help you do more than that, but right now I’m clarifying only that.