Assuming this is a pre-formatted USB stick, it probably lacks LVM information. One solution is to get DFSee from Hobbes (if you don't already have it); start DFSEE (DFSOS2) from the Desktop menu; DfSee will display the drives on your system, uncluding (hopefully) the USB stick
From the menu choose Edit/edit LVM info; choose your USB stick; in the pop-up window, assign * as the drive letter; click OK, then File/Exit. This, if I've got it right, usually works.