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From: olson@harvard.UUCP (Eric Olson)
Newsgroups: net.micro.mac
Subject: Re: Hardware Write Protect: Fact or Fiction?
Message-ID: <669@harvard.UUCP>
Date: Thu, 30-Jan-86 18:11:56 EST
Article-I.D.: harvard.669
Posted: Thu Jan 30 18:11:56 1986
Date-Received: Sat, 1-Feb-86 06:27:06 EST
References: <594@sigma.UUCP>
Reply-To: olson@harvard.UUCP (Eric olson)
Organization: Aiken Comp Lab, Harvard
Lines: 32
Keywords: Larry Rosenstein, please respond

Here's part of the answer to the Write-Lock stuff for Sony drives:

I'm sitting here with a naked 400K drive in front of me.  What follows is
fact (I certainly hope!):

- The drive contains two arms, one on each side of the bottom of the disk
slot (its easy to see the one on the right through the front of your Mac).
- The arm on the right detects disk insertion.  If you press it, the Mac
will tell you it can't read the (nonexistent) disk.  I've used this to 
reformat disks.
- The arm on the left detects write lock.  No light shines THROUGH the
hole in a write-locked disk, the arm is just allowed to be higher up, 
"punching through"  the hole.  Sometimes you get a disk with a faulty
write lock tab (tab inserted backwards), and since the tab has an indent
on one side (supposed to be the side away from the arm; the top), the disk
is always write locked (it is possible to turn the tab over).
- The arms (both of them) are NOT connected to microswitches, as one might
expect, but to PHOTOINTERRUPTERS (an LED and a photodiode pointing
face-to-face with a slot inbetween).  So, if it is possible to turn the
LED on the left arm off, the disk would think it was write-enabled.

I also have a naked 800K drive (which, by the way, is much smaller and
has a nifty wafer motor to turn the disk 'round).  It has a microswitch
on each side, in place of the the arm/interrupter arrangement.  So there
is no fooling the 800K drives via turning a LED off, but there may be another
way.

I suspect the remembered experience with Finder 1.1g might just be that
it allows you to change things that the new Finders don't (on write-locked
disks), but then just didn't update the (write-locked) disk.

-Eric.