Check that labels are enabled (or set to "default") for the overall toolbar in the Customize dialog. That setting overrides the one on individual buttons.
Thanks. I looked into it and found out that the Drives toolbar was set to display the labels, but the drive labels still did not appear.
Eventually I discovered that the problem was that at some point I must moved/dragged the Drive buttons onto the Tabbar toolbar, which has its label state set to "off" by default!