How could I find out which drive letter the usb-stick have?

Hello everybody

I try to get the drive letter for an USB-stick by calling FilterGetDosName but it always returned an error.
I use FltParseFileNameInformation in my driver to get the filepath, then I send it to my userapp there I call FilterGetDosName.

The volume name I try to send in to FilterGetDosName is:
\Device\Harddisk1\DP(1)0-0

I shall also say that it all works fine when I have a file that are located on a harddisk.
\Device\HarddiskVolume1\

How could I find out which drive letter the usb-stick have?
Are there some other ways to find out the drive letter?

Thanks
Mattias Bergkvist