My Mac SE/30 Part VII: SCSI2SD Setup #1

In Part VI of this series, I replaced the dead hard drive in my vintage Macintosh SE/30 with a modern SCSI2SD.

Next I’ll need to set up and configure the SCSI2SD to meet my needs. Now, despite the wonderful versatility of the device, its configuration is not for the faint of heart, especially when used in a vintage Macintosh. Thankfully this isn’t the first time I’ve tangled with configuring a SCSI2SD – when I initially restored my Power Macintosh 8600/200, I also used a SCSI2SD instead of a standard hard drive. In fact, I used this exact SCSI2SD!

See Adventures in Macintosh restoration Part VI: Booting up and jacking in for that story.

In short, the power of the SCSI2SD is that it can be set up to emulate one or more virtual drives all on a single SD card. However classic macs (or at least, the official disk tools needed to set up a new drive) are picky about the brands and models of drives they support. So while you can set up your SCSI2SD virtual drives to “spoof” those blessed drives, the configuration software is not very user friendly.

Sectors not files

The biggest annoyance with the SCSI2SD is how it stores these drives on the SD card. You’d expect that it would just use a standard formatted SD card with each virtual drive stored as a separate “image file”, like with the Floppy Emu. Instead the SCSI2SD writes directly to a range of raw sectors on the SD card, ignoring anything else about how the card is set up.

So if you pop the SD card into your PC, you won’t see obvious files like “Disk 1.dsk” or anything. In fact the default SCSI2SD configuration is a 2 GB virtual drive written to sector zero at the very beginning of the card. Which is actually kind of a problem.

Now, this is a simplification, but sector zero is usually where the filesystem information (the names and locations of each file) is stored. A PC will look at sector zero for a filesystem it understands, which these days is typically FAT, FAT32, exFAT, or NTFS. It won’t recognize a vintage filesystem that may be there. So when it doesn’t find the data it expects, it’ll assume the disk needs to be formatted, and will prompt you to do so.

Accepting that prompt will, in all likelihood, completely corrupt or destroy the data on your card. Yikes!

With this setup, the best you can do is always remember to say no to formatting, which will preserve your data, but also severely limit how you can interact with it. You can still use modern disk imaging utilities to back up the entire SD card to an image file on your computer. However you won’t be able to easily separate out multiple virtual drives, let alone transfer individual files in or out of them.

A better setup

While complete disk images are useful (and better than no backup), I’ve found a better setup which makes life a little easier on myself.

The trick is to put a modern filesystem on the card, starting at sector zero, but not take up the entire disk with it. If I only create a small 32 MB FAT partition, and leave the rest of the disk “unallocated”, then when I stick the card into a PC, it’ll see that small filesystem and not prompt me to format anything. The PC will ignore all of the unallocated space after the partition, so that’s where I’ll configure the SCSI2SD to write its data.

By keeping them separated in this way, neither the PC nor the SCSI2SD will ever interfere with one another in regular, everyday usage. You can add or remove files on that small FAT partition without ever worrying that you’re corrupting the vintage data managed by the SCSI2SD.

Okay, but why only a 32MB FAT drive? Why so small?

While my space needs for this vintage SE/30 are small, I don’t really intend to use the FAT partition for all that much. So there’s no need to take away a bunch of space that the SCSI2SD could be using. In fact, as we’ll see later, I only intend on using the FAT partition to store some useful tools relevant for using the SCSI2SD.

Disk planning for my SE/30

Speaking of my SE/30, my original plan was to have two virtual drives for this machine, one with System 7.5.5, and another with System 6.0.8 for older, “32-bit dirty” applications. For more information about what “32-bit dirty” means, see My Mac SE/30 Part IV: Upgrade Plans.

However, while people may have “dual-booted” back in the day, in my experiments I found that keeping a System 6 setup was completely unnecessary. There’s really nothing I can’t do in System 7 – if I need to run “32-bit dirty” apps, it’s easy enough to just toggle the switch in the Memory control panel and reboot. Actually switching between System 6 and 7 meant toggling the switch and letting each system “rebuild the desktop file” on the disk every time it restarted, which was annoying.

Ultimately, I found it much easier to just keep a set of System 6 images on my Floppy Emu, and plan on booting from floppy if I ever really need to run System 6. With that decision made, the biggest question now was how big of a disk did I need for System 7.5.5?

The maximum size of a classic mac drive (using the vintage HFS filesystem partition) is 2 GB. Realistically, I’ll probably never need that much space on this machine. I mean, the old drive in this machine wasn’t even 0.5 GB and would have been crazy expensive at the time. At my first pass, since I only want to manage one virtual drive, my first instinct is to create a 2 GB disk on a 2 GB SD card.

However, just because an SD card is advertised as being 2 GB, doesn’t mean it can literally store 2,147,483,648 bytes of data. Beyond the regular manufacturer marketing shenanigans of memory bytes vs storage bytes, different SD cards from different manufacturers may have different numbers of actual bytes available.

Which means, rather than use all of the unallocated space for my virtual drive, it’s safer for me to make the disk smaller and leave a decent safety buffer at the end. That way if my card dies and needs to be replaced with a new one that happens to be slightly smaller, I won’t have to worry about my backups not fitting.

Anyway, in the end, I decided to set up my 2 GB SD card like this:

Partition NameDescriptionTypeSector OffsetSize (Sectors)Size (Bytes)
SCSI2SDBackup UtilsFAT065,53633,554,432 (32 MB)
Unallocated65,5362,0481,048,576 (1MB)
Macintosh SDSCSI 1HFS67,5843,670,0161,879,048,192 (1.75 GB)
Unallocated3,737,600184,32094,371,840 (90 MB)
Total3,921,9202,008,023,040 (1.87 GB)
My partition setup for a 2 GB SD card (1 sector = 512 KB)

I have two main partitions: the 32 MB FAT drive at the start to satisfy modern machines, and a 1.75 GB HFS drive for my SE/30. I’ve also put a small 1 MB buffer between the partitions for a little extra safety and separation, and finally there’s some 90 MB of buffer at the end of the disk.

With that plan in mind, stay tuned for Part VIII, where I’ll walk through the steps of actually setting up the SD card in this fashion.

/jon

Want to read from the beginning? Start at Part I.

P.S. My thoughts on how to plan, execute, and document setting up my SCSI2SD was largely influenced by these two blog posts: SCSI2SD: Using a SCSI2SD adapter to setup your 68k Macintosh and Apple IIe Card and SCSI2SD: How I have my SCSI2SD setup for my Apple IIe card in my LC 475. Enormous thanks to the folks at savagetaylor.com for all of their detailed posts on classic macs and the SCSI2SD.

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