I've done some expermenting with 3DS linux whatever build that @AlexBg uses has full sd rw support on all 3DS systems in cluding partition the sdcard with fdisk however only has 1024K of ram while modern builds have 2024K of ram available. I also discovered the linuxfw.bin next
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the linuxfw.bin has to be the binary for handling the mounts. if someone could fix/compile the linuxfw.bin file and fix the sdcard rw on the modern linux builds from the github the mounting sd rw would work prob.
For modern builds with the released linuxfw.bin we get permission denied on mount -w on all volumes because of this doesn't matter the volume.
that linuxfw.bin (actually called arm9linuxfw.bin) it the firmware to make Linux communicate between the ARM9 and ARM11 processors, nothing to do with the mounts, and also the OS have almost all 128MB of RAM avialable to use