Protect the drive letter of disconnected network drive

Hola,
I want to connect a network drive in Win7, but in a way that when it's disconnected, it's drive letter should not become free for other drives.

I've tried to map a shared folder on the network several ways: simple right click/connect, subst, registry ((HKEY_CURRENT_USER\Network\X))) but each way, when the folder is unreachable, and eg. I plug in a USB drive, the stupidest Windows-thing happens: the share's label and drive letter gets assigned to the removeable drive.

I know I could assign a further letter in the alphaber (like W: or Z but that would be inconvenient and also not a complete solution (since in theory they could also be assigned to a different drive).

Is there a proper solution to create a truly persistent network drive?

I would create a desktop shortcut to the mapped drive, which should reserve that drive letter in Windows 7 much the same way that A: and C: are reserved. THIS explains creating that mapped shortcut.