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Question: I'm wondering why I can't create a directory. If I try to create a dir.

  mkdir /mnt

I get this,

  nsh: mkdir: mkdir failed: 2

although if I do this it creates both directorydirectories, mnt & and sda

  mount -t vfat /dev/mmcsd0 /mnt/sda

Answer:  This is because the top level directories are part of a pseudo-filesystem – like the Linux proc/ or sys/ file systems.
  But the NuttX pseudo-file system begins at the top level /.

What that really means is that you do must have CONFIG_DISABLE_PSEUDOFS_OPERATIONS selected.  Because you can normally create directories in the pseudo-filesystem with not problem:

NuttShell (NSH) NuttX-9.0.0
nsh> mkdir /mnt
nsh> ls
/:
 dev/
 etc/
 mnt/
 proc/
 tmp/
nsh> ls mnt
/mnt:
nsh>

But lets assume that you do have operations on the pseudo-file system disabled.  Why doesn't it work?   There is no real media there so you cannot create a file there or create any directories there.
  The mount command is special, it knows how to create mount points in the pseudo-file system.

The pseudo-file system is just a tree structure in RAM.
  It serves two purposes:
  (1) you don't have to have a real file system to use NuttX.
It comes up out-of-the-box with usable (but limited) pseudo-file system.
  That allows a little more civilized programming environment on even very resource limited MCUs.
  And (2) this pseudo-file system is a place where all special NuttX files are retained:
  Character drivers, block drivers, and mount points.

The NuttX top-level pseudo-filesystem creates the illusion of directories and provides a consistent, seamless semantic for interacting with mounted file systems.
  If there is a file called hello.txt in your volume mounted at /mnt/sda, then:

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This is a little different from Linux: Linux always has to boot up with a real file system – even if it is only a initrd RAM disk.
In Linux, these special files (links, drivers, pipes, etc.) reside on real media and can reside in any Linux-compatible filesystem.

Normal mkdir can only work if there is a real filesystem at the location.
  There are no real directories in the psuedo-filesystem.
  The pseudo-filesystem does support nodes that look like directories and have some of the properties of directories (like the node /mnt mentioned above).
  But this is really an illusion.I suppose that one could add

If CONFIG_DISABLE_PSEUDOFS_OPERATIONS is not enabled,  then NuttX adds the capability to create new, empty nodes in the pseudo-filesystem using 'mkdir' –
but I am not sure that this would really serve any purpose other than , completing the illusion.

[On the other hand, all directories are really an _illusion_ in a way and I suppose that in that sense these nodes the pseudo-filesystem are just as _real_ as any other directory.]

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There are a few other special NSH commands like mount that can change the pseudo-filesystem. Like losetup,{{ mkfifo}}, mkrd, umount, etc.
In fact, these commands only work in the pseudo-filesystem.
  Try them in /mnt/sda... they won't work.

But none of the normal commands that modify files or directories will work in the pseudo-filesystem: mkdir, mv, rm, rmdir.
  These all require real media.
  They will not work in the psuedo-filesystem, but will work in /mnt/sda.

And trying to pipe to something in the pseudo-filesystem will also fail
.  You cannot do this, for example:

  NuttShell (NSH) NuttX-6.20
  nsh> cat "Hello, World!" >/hello.text
  nsh: cat: open failed: 22
  nsh>

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