Using dd to image CD ISO files to magneto optical disks - issues with 2048 byte disk not found on 512 byte disks

MrFahrenheit

Well-known member
Hey all,

I have in my hands an A/UX 3.0.1 magneto optical disk, which I imaged over back in 2021 from the original CD ISO. That worked great, but wouldn't mount (for obvious reasons) until I got a SCSI disk driver onto it.

I could start up my Mac using that MO disk and boot into A/UX successfully. Doing the same process, I built a Jaz disk containing the A/UX 3.0 CD installer, and I could install A/UX from that Jaz cartridge, no special CDROM drive needed anymore.

Great! I honestly don't recall the process I did to accomplish that, back in 2021, and so I'm trying to replicate what I did, and so far it's been failures for about 20 hours of working on it.

Until just now. Sort of.

Let me explain the process. I took a good ISO file imaged from original A/UX install CD, and fed it through dd on my G5 running Mac OS X 10.4 Tiger. I used:

# diskutil list

Which gave me the device drive numbers for each device in the system. I "mounted" the A/UX ISO using Disk Utility (making sure the image was locked before mounting), and it appears as disk3. My MO drive is a FireWire one, on disk2. I used a 540MB MO disk for this experiment:

# sudo dd if=/dev/rdisk3 of=/dev/rdisk2

This images the entire contents of the ISO and its partition structure over to the 540MB MO disk.

Once complete, I unmounted it, and walked over to my Quadra 650, and inserted it. Launched FWB Hard Disk Toolkit 2.5.3 and did 'update driver' which placed the FWB driver onto the MO disk, allowing it to work under normal conditions. With the disk inserted, I rebooted, and FWB was now the driver for that disk. Set the Startup Disk to boot from the Mac partition on that A/UX disk, and rebooted. So far so good, it reboots just fine.

A/UX is a bit of a weird creature, and you first boot into Macintosh System 7.1, and there's an A/UX Startup app which bootstraps the A/UX kernel into memory, dumps the remaining contents of memory, and starts up using A/UX. All good so far. I was now in A/UX, booted off a MO disk, without even installing it. This is what I wanted... because eventually I want to make it so I can install FROM the MO disk to something else, so those Apple CD drives won't be necessary.

Here's where the issue seems to surface: I've been using 540MB 3.5" MO disks, which are 512 bytes per sector. These disks are rare, hard to find, and expensive. I've only got a handful. I've got lots of 640MB MO disks, but they're 2048 bytes per sector.

No problem, right? Well, not so. If I image the same CD ISO to a 640MB MO disk, HDT on the 68K Mac fails to see all of the partitions, and it appears as 'damaged'. Nothing I do can make it work, at all.

# sudo dd if=/dev/rdisk3 of=/dev/rdisk2 bs=2048

Note, I had to add bs=2048 otherwise it won't image, saying there's an error.

So I tried something different: I created a 640MB MO disk with the same partition layout as the CD ISO, and then dd imaged individual slices overtop of their corresponding slices on the MO disk. That worked okay and appears fine, boots into the Mac OS part, but A/UX startup failes, likely because it's expecting the A/UX partitions to be at a particular spot on the MO disk, and they're not. The reason they're not, is the hard disk driver wants to install before the A/UX partitions on the MO disk, but on the 540MB working disk, I've installed the FWB driver and it is after all of the partitions, so they're exactly where they should be.

I don't know if that's the reason A/UX won't startup off the 640MB disk or because there's a 2048 byte vs 512 byte issue. Of note, when I imaged to Jaz it also works perfectly, and Jaz is also a 512 byte per sector disk format.

Anyone want to play around with me on this? Anyone want to toss in some insight?
 
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