SCSI diagnosis: Easiest way to make a read-only boot disk (ZuluSCSI or MacSD)

[David Cook]

Over the years, I have accumulated a handful of Macintosh IIsi computers with partially working SCSI that end up corrupting the hard disk images. No other computers have suffered the same fate, and I have plenty of IIsi computers that work perfectly. So, I am fascinated to determine what exactly makes the IIsi so vulnerable.

Before I hook up my oscilloscope and logic probe, I need to have a disk image that is read-only (to prevent corruption) and contains the various utilities I'll be using. The obvious method is to burn a CD and then read that CD back as an iso image. Then, place that iso image on my MacSD or ZuluSCSI. I feel like there must be a simpler method.

What is the easiest way to make a read-only volume on MacSD or ZuluSCSI?

Bonus: Any tips on diagnosing SCSI issues? (I am aware of termination and the fact that term power is missing on the IIsi)

- David

 

[zigzagjoe]

ZuluSCSI can do a small floppy-sized image in ROM on the microcontroller (ie. no card needed).

Not sure about read-only HDD images, though.

 

[kerobaros]

This may be a silly idea, but have you tried marking the hard drive image as a read only file on the SD card while the SD is attached to your modern system? No idea if the ZuluSCSI/MacSD firmware will respect that filesystem flag, but that's the first thing I'd try.

 

[ymk]

No idea if the ZuluSCSI/MacSD firmware will respect that filesystem flag,

MacSD will not, nor will it read the lock slider on the physical card.

You can mount an HD image as a CDROM, as long as its size is a multiple of 2KB.

I'm fairly certain there's a flag in the HFS header for marking a filesystem read only.

I recommend keeping a backup copy of your HD image, restoring as needed.

The Rominator II could also be useful for building an immutable, bootable toolkit.

 

[David Cook]

ZuluSCSI can do a small floppy-sized image in ROM on the microcontroller (ie. no card needed).

I like this idea. I was hoping to create a larger disk with System 7 and some SCSI utilities, though.

This may be a silly idea, but have you tried marking the hard drive image as a read only file on the SD card

MacSD will not, nor will it read the lock slider on the physical card.

Yes, I remember reading somewhere that the read-only attributes in the SD card are not honored. I think this would be a nice enhancement though.

You can mount an HD image as a CDROM, as long as its size is a multiple of 2KB.

Will this be write-protected by the MacSD? That is, will it ignore attempted writes to a 'CDROM' by a Mac with a defective SCSI system?

I'm fairly certain there's a flag in the HFS header for marking a filesystem read only.

I was thinking about this. It may help the OS deal with the fact that the system is read-only, but may not stop spurious hardware.

I recommend keeping a backup copy of your HD image, restoring as needed.

Absolutely! Fortunately, I always consider the images on the SD card as 'secondary'. They are never copied back over the original image on the modern computer. I made that rule when I first started repairing old Macs -- guess why. : )

The Rominator II could also be useful for building an immutable, bootable toolkit.

This is also a cool idea, but doesn't exercise the SCSI bus. To boot, test, and access tools, I have successfully used FloppyEmu in HD20 mode on these machines. Running MacTest Pro passes the memory and logic board tests. The recreated Apple TechStep also detects bad SCSI (at least on the IIsi I tried it on).



After I choose a method for a read-only SCSI disk, diagnosing the SCSI problems is going to be interesting. On one of the IIsi computers, I even desoldered the SCSI chip to check for bad traces. I toned out all the traces successfully and then resoldered the chip without luck.

My theories:
1. The SCSI chip is bad?
2. The traces tone out but are actually marginal?
3. The serial chips (which also go the SCSI chip) or their traces are bad and are interfering with the SCSI chip?

 

[jmacz]

Absolutely! Fortunately, I always consider the images on the SD card as 'secondary'. They are never copied back over the original image on the modern computer. I made that rule when I first started repairing old Macs -- guess why. : )

I've been attempting to do the same as I recently had an issue where a seemingly ok internal SCSI CD-ROM drive was actually partially faulty (capacitor leak related) and due to how it was behaving on the SCSI bus caused corruption to the drive images hosted on an SD card (ZuluSCSI).

But the issue is that I actively use my Macs and thus the contents on the SD card are constantly changing and the newest... which means I have to remember to back those images up from time to time. Which is annoying as my OCD causes me to use an internal ZuluSCSI (instead of an external) which on a Quadra 800 is annoying to get to, and the few micro SD extension cables I have found are not made well and tend to not detect every few boots due to tolerance issues on the pseudo micro SD card (you can wiggle it and it detects/doesn't detect).

 

[ymk]

Will this be write-protected by the MacSD? That is, will it ignore attempted writes to a 'CDROM' by a Mac with a defective SCSI system?

I'll have an update ready soon to prevent this.

This is also a cool idea, but doesn't exercise the SCSI bus.

Not directly of course, but with a bootable system and utilities on a Rominator, you could give a SCSI device the business without putting your boot volume at risk.

 

[David Cook]

I'll have an update ready soon to prevent this.

That is awesome!

 

[rabbitholecomputing]

Over the years, I have accumulated a handful of Macintosh IIsi computers with partially working SCSI that end up corrupting the hard disk images. No other computers have suffered the same fate, and I have plenty of IIsi computers that work perfectly. So, I am fascinated to determine what exactly makes the IIsi so vulnerable.

What is the easiest way to make a read-only volume on MacSD or ZuluSCSI?

@David Cook We just fixed a long-standing bug in the ZuluSCSI code base related to this, and I've cut a development preview release. Now, if you set the read-only permission flag on an image, it is properly exposed as read-only when the MODE SENSE command is issued by a host machine.

This will be incorporated into a future release, but for the time being, if anyone wants to try it out, you can download the appropriate firmware from https://github.com/ZuluSCSI/ZuluSCSI-firmware/releases/tag/v2025.03.18-preview

 

[David Cook]

@David Cook We just fixed a long-standing bug in the ZuluSCSI code base related to this, and I've cut a development preview release. Now, if you set the read-only permission flag on an image, it is properly exposed as read-only when the MODE SENSE command is issued by a host machine.

This will be incorporated into a future release, but for the time being, if anyone wants to try it out, you can download the appropriate firmware from https://github.com/ZuluSCSI/ZuluSCSI-firmware/releases/tag/v2025.03.18-preview

Very cool!

Despite being told it is read-only, if the host tries to write anyway, will the write succeed? I ask only because I know the SCSI is faulty on these Macs, so I can't rely on them behaving themselves.

 

[LaPorta]

Why not start up another machine with some other disk, hook up the MacSD/ZuluSCSI to that machine, and in Mac OS using a utility, make the disk read-only?

 

[rabbitholecomputing]

Despite being told it is read-only, if the host tries to write anyway, will the write succeed? I ask only because I know the SCSI is faulty on these Macs, so I can't rely on them behaving themselves.

Nope, it will fail, as it should Whether the OS handles that properly or not is up the OS.

 

[David Cook]

Nope, it will fail, as it should Whether the OS handles that properly or not is up the OS.

Awesome