So that the system always assigns the same letters to the partitions at each connection of the Hard Disk in Wndows 7 Ultimate, you must proceed as follows:
click on Home - Control Panel - System and security - Administrative tools - Computer Management.
In the window that opens, click on Disk Management. In the list at the top right, you will find the icon of one of your connected drives, for example My Book (N :). Right click on it, select Change drive letter and path. and then, in the window that opens, click on Change ..
In the next window click N (with the down arrow symbol) and select a letter of your choice, using the trick of selecting a letter far ahead in the alphabet (for example X).
Windows, in fact, assigns the first letter it finds free to the Drives. If one of the first letters of the alphabet were chosen, there would be a risk of finding it already occupied when connecting the disk. Click twice on OK, and repeat the procedure for all discs.
From now on, each disk will be assigned, at the moment of connection, always the same letter chosen by you.
How to format an external hard drive