I have a RISC OS 4.39 RiscPC with a Castle USB card and Compact Flash card reader attached. This uses Castle‘s !SoftSCSI to access the flash cards.
 
For a long time, I have been frustrated by its inability to mount any flash card larger then 2 GB in size. I really wanted to use a single flash card, to back up my 40 GB hard drive, as having to use a pocket full of 2 GB cards is a real pain.
 
I had been put off experimenting with Jeff Doggett‘s FAT32FS before, because I had read somewhere that it only works with the USB stack on the Iyo.
 
But, as it‘s the only solution, I did have a go at trying to get FAT32FS working on my RiscPC, recently. And, luckily, it turns out that the USB podule card on the RiscPC, is sufficiently similar to the one in the Iyonix, that it does, in fact, work.
 
It did take a bit of experimentation, however, to work out how to do it.
 
For a while, before I had figured it out, I was finding that I could mount a larger flash card and save files to it, but that I couldn‘t get it to re-mount the drive, after I had switched the machine off and on again.
 
Effectively, a larger flash card could be formatted, and used as a scratch disk for a single session, but not for long-term backup — which is exactly what I wanted to use it for !
 
There are two changes you need to make to your system, to get it to work.
 
The first, is that you need to install the Fat32FS module in the PreDesk directory of your !Boot application — !Boot.Choices.Users.Single.Boot.PreDesk.
 
The second, is that you have to edit the end of the !Run file, inside your !SoftSCSI application, to say -

FAT32FS:Mount -v :0

This tells !SoftSCSI to mount the flash card, using the FAT 32 driver, rather than its own internal one. You can now mount a larger flash card, just by double-clicking on the !SoftSCSI icon.
 
But remember to Dismount it from the drive icon, once you have finished with it. This reduces the chances of the drive accidentally becoming corrupted, by something like a sudden system freeze.
 
If you do accidentally corrupt the drive, you can always re-format it, using the author‘s FAT32Format program. You need to create an Obey file, like this -

ADFS::HardDisc4.$.Fat32Format -f32 :0

to format the disc in drive 0.
 
You could set your system to load !SoftSCSI automatically, on boot up, but my Epson scanner, which is also plugged into the Castle USB card, will not work once !SoftSCSI has been loaded, so I only launch it when I want to back something up.
 
Fat32FS offers two huge advantages over just !SoftSCSI on its own — not only does it allow you to break the 2 GB ceiling on disk size, but my impression is that read and write speeds are about five to ten times faster. And that makes a very worthwhile upgrade for a RiscPC user, in anybody‘s book.