One solution is to use the label assigned to each device. The full name of drives appears as CRUZER (J:) where J is the assigned drive letter and CRUZER the label. You can change these labels if you press <F2> to rename the drive while browsing My Computer (This PC). Choose the labels to conveniently distinguish your external storage devices, as they appear on e.g. the folder tree pane. When it comes to xplorer² drivebar, which only shows the drive icon, the label appears (oddly) on the status bar as you hover the mouse over the drive icons.
For more visual impact and easier identification of your thumb drives, you can assign a custom icon. For most folders, you can replace the standard yellow folder icon using the property page (press <F12> or right click on the folder), then Customize tab. However if you try this for a drive you will find that the change icon button is missing. Assigning a thumb drive icon is a bit awkward but not too hard: we just need to create (or edit) a file called AUTORUN.INF placed in the root folder to specify the desired icon. Here is a step-by-step illustration:
Another option is to copy the above text, click on the J: folder pane then hit <Ctrl+V> to paste it in a text file in one stroke—then rename it to autorun.inf
Now the J: icon really stands out even on the drivebar!