Editing IBM Personal Computer AT (section)

== Problems ==
In addition to the unreliable hard disk drive,<ref>IBM's official 1986 response to "What percentage of the 20 MB drives in PC ATs have failed?" was "We consider that information to be confidential. However, based on the several customer surveys on the AT that we have conducted for IBM, an overwhelming percentage of AT owners tell us they're satisfied with the system." (questions on page 110, answers on page 111, PC Magazine, April 29, 1986).  The article's opening sentence, which reads "If you own an IBM PC AT and your hard disk hasn't crashed yet, don't worry -- it probably will."  {{Cite web |url=http://www.highbeam.com/doc/1P2-3760999.html |title=Archived copy |access-date=2010-06-30  |archive-date=2012-11-04  |archive-url=https://web.archive.org/web/20121104075857/http://www.highbeam.com/doc/1P2-3760999.html |url-status=dead }} was described as "a rarity in computer journalism" by the Chicago Sun-Times http://www.encyclopedia.com/doc/1P2-3760999.html and the Sun-Times called it a "badly flawed 20-megabyte" disk drive.</ref> the high-density floppy disk drives turned out to be problematic.  Some ATs came with one high-density (HD) disk drive and one double-density (DD) 360&nbsp;KB drive. High-density floppy diskette media were compatible only with high-density drives.

There was no way for the disk drive to detect what kind of floppy disk was inserted, and the drives were not distinguished except by an asterisk molded into the 360&nbsp;KB disk drive faceplate. If the user accidentally used a high-density diskette in the 360&nbsp;KB drive, it would sometimes work, for a while, but the high-[[coercivity]] oxide would take a very weak magnetization from the 360&nbsp;KB write heads, so reading the diskette would be problematic.

Conversely, the high-density drive's heads had a track width half that of the 360&nbsp;KB drive, so they were incapable of fully erasing and overwriting tracks written by a 360&nbsp;KB drive. Overwriting a DD disk that had been written in a DD drive with an HD drive would result in a disk that read on an HD drive, but produced read errors in a DD drive.  Whereas a HD read head would only pick up the half track that drive had written, the wider DD read head would pick up the half-track written by the HD drive mixed with the unerased half-track remnant of the track written earlier by a DD drive. Thus, the DD drive would end up reading both new and old information together, causing it to see garbled data.