Hi again,
From your answer, I can’t filter the exact problem, sorry.
If the Device is trying to assigning to an existing drive letter or your Windows portable Device driver is corrupted.
I have three different solutions to try.
First the easiest solution: Go to Windows
‘Fix it’ for solving problems with USB devices.
http://windows.microsoft.com/en-US/windows-vista/Tips-for-solving-problems-with-USB-devicesIf that doesn’t work try to assign a different drive letter.
Open the disk manager (diskmgmt.msc) find the USB and assign a different drive letter.
And If this also doesn’t help, Try to reinstall the WPD.
Open de device manager (devmgmt.msc) and remove the USB corrupted device.
And then (with the right mouse click on your computer name), scan for changed devices..