Rower85 said to do this...
Open Run and type "Diskpart"
type "LIST VOLUME"
note witch volume number that you want to autoassign on boot
Create a textfile on C:\ named: diskpart.txt
Put this in the textfile:
select volume "drive number"
assign letter="drive letter to assign"
(in the " " brackets, input your stuff, also you do not need the "")
Open Run and type notepad
Put this in:
diskpart /s c:/diskpart.txt
click File > Save As...
locate C:\ and change File Name to "diskpart.bat" then change Save as type: to All Files and press Save
Now go to Task Scheduler in the Admin section on the Control panel.
Make a new folder in the "Windows" sub folder named "Diskpart"
rightclick the "Diskpart" folder
click "Create Basic Task..."
name it to "Diskpart" and click "Next >"
select "When the computer starts" click "Next >"
select "Start a program" click "Next >"
click "Browse..."
select the "Diskpart.bat" file you saved on C:, click "Next >"
select "Open the Properties dialog for this task when I click Finish" click "Finish"
Properties should come up, now select "Run with highest privileges"
click "OK"