n USB pen drive is drive and media in one. Therefore the drive letter comes and goes with it. In contrast a card reader stays attached and only the card is plugged and unplugged, so the drive letter(s) stays.

Because it's drive and media in one, each time you plug a pen drive for the first time the Windows plug'n play starts up a assigns the first free drive letter. When the USB has no USB serial then this happens again if it's atteched to a differenct USB port.

To get rid of the 'first free letter assignment' I wrote the USB Drive Letter Manager which works under Windows 2000 and XP.