Just saw this note whilst browsing the help on Drive Maps in Group Policy Preferences:

You can use a Drive Map preference item to configure the visibility of a physical drive rather than a mapped drive. To do so, select the Update action, leave the Location field blank, select the drive letter of the physical drive, and then configure the Hide/Show this drive and Hide/Show all drives options.

So, to hide “A” drive (in this example), you’d configure the dialog box as follows:

I didn’t know this was possible. This may come in handy one day, if you want to hide something like an OEM partition that has had a drive letter assigned.