ST-00 was the name of the floppy from where Sound/Noise/ProTracker was booted, while the samples floppies labels started from ST-01. ST-00 is still required by Protracker even if installed on a harddrive, this assign is usually added in the user-startup file by the PT installer, or made by clicking a provided assign icon. You can do it manually, too, with "assign ST-00: pathwherePTis/".
Varthall