external disk drive (problem/solution)

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From: info-mac@uw-beaver (info-mac)
Newsgroups: fa.info-mac
Subject: external disk drive (problem/solution)
Message-ID:
Date: Tue, 27-Nov-84 04:38:52 EST
Article-I.D.: uw-beave.2337
Posted: Tue Nov 27 04:38:52 1984
Date-Received: Wed, 28-Nov-84 03:51:22 EST
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Organization: U of Washington Computer Science
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From: [email protected] (mknox)

One of those little problems for the ulcer prone:

I recently added an external 3.5 drive for my MAC. It worked for a few days
just sitting by the MAC. Then I got industrious and built this nice little
set of shelves, with a 'perfect' place for the drive. Just to the left of the
MAC, and up about nine inches above the table where the MAC sat. I also
rotated the MAC to the right slightly, so that it pointed more toward where
I sit.

My external disk drive stopped working. Hummm, whirrr, no read/write. Just
'bad disk' messages. I moved the drive for a better look, AND IT STARTED
WORKING!!! 'Ah Ha!' said I, 'It is a bad cable.' But the cabled checked
good.

Finally I got an idea. Out with an old stainless steel cookie sheet. Place
it between the MAC and drive. Perfect operation. No cookie sheet (and the
drive back on its shelf), can't read a thing.

Solution: rather than design a Faraday cage for the drive, I added another
little shelf about 4 inches lower, and about 4 inches further away from the
MAC. No problems.

Cause? Probably RFI from the flyback xformer in the MAC (second harmonic
is near the bit-rate on the drive). It only causes failure in close proximity,
within a fairly narrow range of angle and position. But the failure is
VERY repeatable. If you are having trouble, try moving your drive a little
(point it away from the MAC seems safe for a test).

I haven't seen a design like this since the external drive on the old
TRS-80 Model-I (same problem).