Path: utzoo!attcan!uunet!cs.utexas.edu!usc!apple!mattd
From: mattd@Apple.COM (Matt Deatherage)
Newsgroups: comp.sys.apple
Subject: Re: Removable Hard Drives
Message-ID: <36632@apple.Apple.COM>
Date: 19 Nov 89 19:00:29 GMT
Organization: Apple Computer Inc, Cupertino, CA
Lines: 63

unknown@ucscb.UCSC.EDU (The Unknown User) writes:
>
>	Well, the engineer explained it all to me, although I've forgotten
>much of it now, so the following simple explanation is probably wrong in
>many respects. At AppleFest, the drive could only give back one of four
>responses if access is made. Of those four, only an I/O error was appropriate
>when a cartridge was taken out (under any circumstances from what I
>remember). He showed me what happened under System 5.0. It just opened
>a window with 0 items if you took out the cartridge and still tried to open
>the device. He said that he was working on a SMARTPORT version of the
>cartridge drive which would be much more intelligent and allow on-line
>dismounting of cartridges (thus I could do what I want to do!)..
>
>unknown@ucscb.ucsc.edu

[lots of stuff deleted to save bandwidth]

Unless CMS supplies a GS/OS loaded driver for their card, which they currently
do not, they get a generated driver (GS/OS makes one up for them).  They ge
the highest level of generated driver they can (if their card supports Extended
SmartPort, they get that; else they get SmartPort, or if they don't support
that, they get ProDOS Block Device).

A ProDOS Block Device has four possible return values when called.  They are,
(without numerical equivalents):

*  No Device Connected
*  I/O error
*  Write Protected
*  Device off-line

It is the fourth code that should be returned for a removable system to let
GS/OS know the device exists but there's no media in it.  That's how the 3.5"
drives work.  That's how the CD-ROM works (and it's SCSI).  That's how every
removable device works.  If they didn't return "device off-line", I can imagine
they were having some problems.

The SCSI Manager also allows for removable devices (obviously, since the CD-ROM
works with it just fine).  However, when the Manager makes a SCSI call to see
if the device is removable [Note:  I'm just guessing here; I know it figures
out somehow that devices are removable and I seem to remember some kind of
SCSI inquiry that allows this determination], they have to respond correctly.
During development of 5.0, we had problems with some hard drives not working
with the SCSI Manager; it turned out that the drives in question wer
responding to SCSI inquiries with totally bogus values (such as "I don't 
support reading or writing").  The Manager has to assume a basic set of
characteristics (like all devices must be read-enabled) for GS/OS to work, so
that's what it does.  If this removable drive returns bogus SCSI values, then
the Manager probably assumes a default set of characteristics, including the
(likely) chance that it is not removable.

I've seen removable hard drives work with 5.0's SCSI Manager in our lab.  I
know it's possible.

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