Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site uw-beaver Path: utzoo!watmath!clyde!cbosgd!ulysses!mhuxr!mhuxt!mhuxj!houxm!vax135!cornell!uw-beaver!info-mac From: info-mac@uw-beaver Newsgroups: fa.info-mac Subject: Re: Programmer's switch Message-ID: <287@uw-beaver> Date: Fri, 21-Dec-84 21:15:42 EST Article-I.D.: uw-beave.287 Posted: Fri Dec 21 21:15:42 1984 Date-Received: Sun, 23-Dec-84 00:35:14 EST Sender: daemon@uw-beaver Organization: U of Washington Computer Science Lines: 33 From: olson@harvard.ARPA (Eric Olson) I read a good theory in INFO-MAC a while ago about why external drives sometimes start trashing disks. The programmer's switch/power off issue may seem to help, but I think it helps by side-effect only. To summarize: The external drive sits in some random position outside the Mac. When the mac reboots (via power cycle or programmer's switch) it resets the video controller (possibly) causing a massive collapse of flux in the flyback transformer (or some other transformer). If the read/write head in the external drive is in the path of this field, its head could become semi-permanently magnetized. It will then trash disks inserted because it writes all over them with a (semi-permanent) magnet. To cure this problem, the suggestion was made to take a disk and keep inserting it in the drive and formatting it. This will put the write-bias current through the head a lot, and hopefully demagnetize the head. Putting a plane of conductive metal (like a cookie sheet) between the Mac and the drive cured the problem [I wish I could take the time to attribute the creator of this wisdom, but I can't look through the archives for that-- leave it at: this isn't original thought]. Considering that the power cycle is probably much more fluxxy than the programmer switch reset, I wouldn't recommend using the power cycle to get the machine to behave better. It's just possible that power-cycle works both ways: sometimes magnetizing, sometimes demagnetizing. Whatever. -Eric [For those of us without cookie sheets, would it be wise to always position the external drive on the RIGHT side of the Mac, farthest away from the fly back transformer(?) --bc]