Mac OS X, like all UNIX-based systems, doesn't really have the clear-cut notion of "drives" the way that Windows does; there's a single file system within which various physical drives are mounted. Your "Volumes" list is really the only way to get this information from Java.
I'm going to move this to our Mac OS X forum, just in case there's a platform-specific trick I don't know about. [ May 12, 2006: Message edited by: Ernest Friedman-Hill ]
EFH is correct; there really isn't any concept of physical drives the way that there is in Windows. UNIX systems abstract the concept to volumes.
The content of the /Volumes "folder" is about as close as you are going to get to a similar concept. But be aware that mounted DVDs, CDs, USB "thumb drives", external firewire/USB2 drives, and even mounted disk images will also appear in this folder. It can even (usually created by accident) have hard-drive folders in it.