Windows 7: stopping usb message external hdd

I Every time i start Windows 7
I got 4 for message screens that my extral hdd is conneting.
How can i disebale it for the specific drive letter.
I had found it but i gant find it anny more.
What i now is that i had to set the alfabet on paper en under the letter that i don't want to see 1
example i don't want to see the e drive
zyxwvutsrqponmlkjihgfedcba
00000000000000000000000
zyxwvutsrqponmlkjihgfedcba
000000000000000000010000

So 10000 is binary an i had to convert it to hex.
But i don't now what registery key i have the change with the hex format.