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SCSI2SD - Configuration of SD Card to use multiple partitions

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First post, by uscleo

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Hey Guys!

I’m having trouble with my SCSI2SD adapter… Does anyone know how to format a 4 GB SD card so that I can use the multiple drive feature for 4 x 1 GB drives in the SCSI2SD? In particular, how to format the SD card in FAT-16 so that it’s partitions can be used as multiple SCSI drives on older computers and the same partitions can be used as partitions to add and remove data from with a modern computer?

I have a 4 GB SD card and I am trying to create 4 1GB partitions. The computer I’m using is an IBM with an older SCSI adapter that can only accommodate 1 GB drives. Where I have gotten to so far is I have used Terminal on my mac to format the SD card into 4 - 1GB, FAT 16 partitions with a Master Boot Record (MBR). The SD card works perfectly in my mac. I then flashed the SCSI2SD to have 4 “drives”, the first starts at sector 2 (sector 1 is the MBR), then the next 3 “disks” to match the sector size of each partition. Then between the 3rd and last partition, there is another extended boot record of one sector, then the last sector starts till the end of the SD card.

Now this works fine in the mac, but when I insert the SD card into the SCSI2SD then install and boot the IBM, I can configure the and detect the SCSI drives in the reference diskette (early BIOS-type utility for IBM’s), and using a MS-DOS install disk, I can run FDISK.

But in FDISK, I can only see the 4 “drives”, with no partitions defined, even though there are 4 partitions of FAT-16 on the SD card.

Once I use FDISK to set the partitions and assign drive letters, I can boot to dos and go to A:\> to C:\>, but it throws an error which is fixed when each “drive” / partition is formatted on the IBM.

After formatting, things seem to work. BUT I can’t get it to boot to the SD card unless I do FDISK / MBR - once I do this however, I cannot access the first partition of the SD card when I plug it in on the mac.

Not only this, but if I copy files to the SD card partitions from the mac, the files don’t appear in the drives on the IBM.

I’m obviously not super well versed in hard drive geometry and settings, but is there something I am missing?

Any help would be really appreciated, thanks guys!

Reply 1 of 2, by Deunan

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uscleo wrote on 2020-01-30, 08:17:

I’m having trouble with my SCSI2SD adapter… Does anyone know how to format a 4 GB SD card so that I can use the multiple drive feature for 4 x 1 GB drives in the SCSI2SD?

In short, you don't. SCSI2SD doesn't need or use any format, it will use the card as a block device and you only specify the first and last sector address of each "disk" that is to be emulated. Up to 4 total.
Whatever is stored on such disk is no business of SCSI2SD - if it's a FAT partition it might be readable by some Windows tools (though very unlikely by just plugging the card into a reader). Custom storage methods will require special tools/apps.
That way SCSI2SD can support any and all SCSI hosts, because the read/write commands are the same no matter what format it uses natively. The downside is the inability to access such data easily on the PC.

I wrote down the start-stop sector numbers for each emulated disk and I mount the card using Linux where you can specify a shift (vs the block device sector 0) to actually start at. So I can mount any of the virtual disks provided the format is known to Linux. Sometimes you have to tweak that starting sector number further to skip any MBR or other stuff (since it's not correct respective to the entire card) but it works. And you can have write access too, unlike on Windows where you'd probably need to dump the card contents first as a binary file and then use some tool like DiskExplorer.

Reply 2 of 2, by jmarsh

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Here's what's happening:
You're putting an MBR/Partition table in sector 0, with the four 1GB partitions. The IBM can't see this partition table because of the way you've allocated the space to the virtual SCSI drives (and if it did it wouldn't be able to use it, since partitions can't span multiple drives) so that's why you need to partition each of them separately with FDISK. Additionally if you want to boot from a drive, it obviously needs a master boot record installed otherwise there's no OS to boot.
When you plug the card back into the mac it sees the partition table at sector 0, but when it looks up the beginning of each partition it finds another MBR/partition table instead of a partition header.
There is a way to do what you want, by adjusting the partitions in the sector 0 partition table to match the values written by fdisk to each individual virtual drive (without changing the ranges assigned to each virtual drive). But you will need to calculate the offsets by hand to take into account the starting offset of each virtual drive.

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