The Drive button in the Open, Save, and Save As dialog boxes has been replaced by the “Desktop” button. When you click the Desktop button, you will see all of the hard disks, floppies, file servers, and folders that you have on your Macintosh desktop. To move to a specific disk, you can go directly to the disk you want, without having to move sequentially from one disk drive to the next.
This is one of those examples where Apple’s change makes sense from a user perspective, but not a logical one. When your computer’s hard drives shows up on the Desktop, it makes sense that you would navigate to the Desktop to access them all, even though it’s bonkers if you stop and think it through for a moment.