the HD20 to achieve much higher transfer rates than products limited to the serial ports. With few exceptions, this along with complete compatibility with the new Hierarchical File System, gave Apple an instant edge over the competition. In addition, the HD20 has a convenient "zero-footprint" design which fit precisely underneath the Macintosh, merely elevating it 3 inches, but otherwise taking up no more desk-space. Specifications Recording Surfaces: 2 Per Drive: 20.7 MB (formatted) No. of Cylinders: 610 Total No. of Tracks: 1220 No. of Sector/Track: 32 Bytes/Sector: 532 (formatted) Total No. of Blocks (Data): 38,964 Spare Blocks: 76 Access Time: Track to Track 10 ms; Average 50 ms; Maximum 150 ms; Average Latency 10.9 ms Rotational Speed: the drive has a rotational speed of 45.73 rotations/second (2744 rpm) and access time of 85 ms. History In 1985, the HD20 was an important step to solidifying the Macintosh as a true business computer and it was eagerly anticipated following its April announcement. Until Apple's introduction a year later of the Hard Disk 20SC, the first SCSI drive they manufactured, the HD20 was the only Apple-manufactured hard drive available for any Macintosh except the Macintosh XL. The HD20 was not compatible with any other Apple computer or other platforms. However, the HD20's unique design and position in the marketplace was quickly outmoded by the advancement of the significantly faster SCSI standard which debuted with the Macintosh Plus in January 1986. Some third party companies offered a SCSI conversion kit which replaced the controller board thus preserving the user's investment in the expensive but proprietary Rodime drive. Apple officially dropped support for the HD20 with System 6 as well as omitting the necessary ROM code beginning with the Macintosh II. Sales of the HD20 continued to support the Macintosh 512Ke which has no other hard drive options, until it was discontinued in late 1987. Apple dropped support for the HD20 in all of its newer Macs, only to find many business users upgrading their older systems needed a way to transfer data from the unsupported drives to the newer Macs. Only Macs with legacy technology and floppy disk ports, which were eliminated entirely from Macintosh computers in 1991, are able to continue to use the older slower technology. Manufactured in significant numbers for almost two years, the HD20 remains as one of the few surviving hard drives usable by a stock Macintosh 512K