To JimS:
The Atari 520ST with external power supply and disk drives used 2 types of drives- SS/DD and DS/DD. The DS/DD drive was an upgrade option offered by Atari. Later on, when the Atari 520FM ST and the Atari 1040 ST was released, the DS/DD disk drive became the standard. Around this same time period the Amiga 1000 computer hit the market. The disk drives found in the Atari ST drives were better quality then the disk drives found in the Amiga 1000 computer. The early Atari ST drives were made by NEC, Epson, and Panasonic. They were built like German Tiger Tanks! :-D