Back in 2008, we told you how to access the slightly convoluted process for changing an assigned drive letter for a USB drive in Vista. Tech blogger Helen Bradley found that the same approach can also help if Vista is flat-out refusing to recognise a drive which you know is working.
Bradley found that two drives she connected to her PC showed up in Device Manager, but weren't accessible in any applications. Reassigning them a drive letter (by searching the Computer Management option, or right-clicking on the Computer icon and selecting Manage) fixed the problem. Windows 7 uses the same method for reassigning drives, so it could be useful there too if Windows is having one of its occasional USB meltdowns.
Drives not recognized in Vista [Helen Bradley]