The Hard Disk 20 was the first hard drive developed by Apple Computer specifically for use with the Macintosh 512K. Introduced on September 17, 1985, it was part of Apple's long awaited solution toward completing the Macintosh Office (a suite of integrated business hardware & software) announced in January 1985. Unfortunately it would be over a year more before Apple would release the file server software AppleShare that would link all of the hardware together. By that time the SCSI interface introduced on the Macintosh Plus in January 1986, would accommodate far faster and more efficient hard drives, rendering the Hard Disk 20 virtually obsolete.

But, in 1985, the HD 20 (as it was known) was an important step to solidifying the Macintosh as a true business computer and it was eagerly anticipated following its April announcement. It contained a 20MB 3.5" Rodime hard disk which provided over 50 times the data storage of the stock 400K disk drive. At a time when the average file size was around 10-20K and due to the vast number of those files the HD 20 could contain, Apple's original Macintosh File System, which did not allow for directories, would have made organizing those files unwieldy. Therefore Apple introduced it with a new System and Finder update which included the brand new Hierarchical File System allowing the user to better organize files on such a large volume. As a result only the Macintosh 512K could access it, the original Macintosh 128K did not have enough RAM to load the new file system. In fact, even for the Macintosh 512K to use the drive, it required an extra file in the System Folder on a special startup disk which added extra code into memory during startup. An ingenious startup routine also allowed the Mac to check for the presence of a System file on the Hard Disk, switch over to it and eject the startup disk. Unfortunately, the HD 20 could not be used as a startup disk directly without first loading the code from the floppy disk drive. With the release of the Macintosh Plus and the Macintosh 512Ke, both containing the upgraded 128K ROM which contained the extra code, the HD 20 could finally be used alone as a startup disk.

Until Apple's introduction a year later of the Hard Disk 20SC, the first SCSI drive they manufactured, the HD 20 was the only Apple manufactured hard drive available for any Macintosh (not including the Macintosh XL). While other hard drives were available on the market, Apple's HD 20 was typically preferred mainly because Apple broke their own development rules when they offered it. Originally the Macintosh was designed with two serial ports which were to give all the expansion needs required of the user. It also included a dedicated floppy disk port for one external floppy disk. Most of the hard drives which were available on the market used the slower serial port to transfer data per Apple's specifications. Apple instead engineered the HD 20 to use the faster floppy disk port, enabling the user to daisy-chain an external floppy disk drive as well as an extra HD 20. 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 HD 20 had a convenient "zero-footprint" design which fit precisely underneath the Macintosh, merely elevating it 3 inches, but otherwise taking up no more desk-space.

However, this unique design was quickly outmoded by the advancement of the significantly faster SCSI standard. Some third party companies offered a SCSI conversion kit which replaced the controller boardthereforepreserving the user's investment in the expensive but proprietary Rodime drive. Apple officially dropped support for the HD 20 with System 6 as well as omitting the necessary ROM code beginning with the Macintosh SE/30.

Manufactured in significant numbers for well over a year, The HD 20 remains as one of the few surviving hard drives a stock Macintosh 512K or 512Ke can use.