Currently Windows (all of the versions) assigns the first available letter to the next removable media you insert into your USB drive. Is there a way to go around this to make Windows assign only letters that are in the interval you want (for example S - Z)?
USBDLM is a Windows service that gives control over Windows' drive letter assignment for USB drives. Running as service makes it independent of the logged on user's privileges, so there is no need to give the users the privilege to change drive letters. It automatically solves conflicts between USB drives and network or subst drives of the currently logged on user. Furthermore you can define new default letters for USB drives and much more. It works on Windows XP to Windows 10.
Windows has a built in tool to assign a persistent drive letter to an external drive. Of course this may not be a complete solution if you have a large number of removable drives and can't assign a consistent fixed set of drive letters to them.
These instructions are for Windows 10, but earlier versions should be similar.
Mount the external drive you want to assign a permanent letter to.
Click the Windows icon and start typing "format". Select "Create and format hard disk partitons, Control panel" when Windows suggests it. The app opens a window titled Disk Management.
Select the external drive from the list, right-click, and select "Change Drive Letter and Paths..."
and from that point, follow your nose!