The SD card that comes with the picocalc is pre-partitioned to include a FUZIX root partition in its last 32MB.
If you want to update that image, you can use dd. There’s a little bit about that in GitHub - wez/FUZIX: FuzixOS: Because Small Is Beautiful and also in the picocalc SD readme docs, but the gist is shown below
MAKE SURE to check which partition you are writing to with the command below, as you will DESTROY whatever is on that partition!
$ sudo dd if=filesystem.img of=/dev/sdb2 oflag=direct bs=8192 status=progress
Please note that, until the most recent build from a few minutes ago, the filesystem image I was publishing in the releases section was a generic image. I switched that to one that is slightly more pico-friendly and that includes a utility that will reboot in flashing mode, which is more convenient than trying to press the bootsel button: picoctl flash.
So I recommend fetching the latest release zip before you flash the image, just to save an extra couple of steps later.