I find that, when I plug in a new USB flash drive, it gets the same letter
assignment as an existing network mounted drive and is inaccessible.  For
example, if the network drive is H: and there is already a G: (not network
mounted), the USB memory will be assigned H: and be invisible to applications.

The only way I've found to fix this is to go into Disk Management, where I
can see and change the drive letter assignment.  If I change the assignment
from H: to T: (for example), then the USB memory is visible as T:.  I can now
dismount it and plug it in at a later time and it will still be seen as T:.  
However, to avoid problems I have to assign different letters to all my USB
devices.  This is not good.

I am using Win x64 (all updates).

Is this expected behavior?