I have a vague memory of reading somewhere that inserting an SD card (or CF, or XD) into a USB adaptor doesn't produce any resulting signal, so it can only be detected by continually polling them.
On a desktop system this would be done by hal(?), which probably isn't running on the Plug.
So, if the adaptor behaves like USB this could be the problem.

That would be a different problem then.  The OP had the root file-system on an SD card, so would (presumably) have had an SD card inserted at boot time.

To talk to a USB device you probably need to talk using the USB protocol (see libusb for details).
I've never come across anything that lets you treat it as a simple character-based device.

You need to have mmcinit in your boot command.  It's not just the kernel that needs to see it, Uboot needs to see it too in order to read the kernel from it.

Possibly not.  3.0.3 was released in front of a live audience at LinuxCon.

...but first it needs to be created as a file system.   I don't think I can just create the device then run "mkfs -t ubi".

I do if I want the rest of the files to be on an external USB drive through a hub.  And I don't want things like /var to be on a solid-state device that can wear out (I've had an SDHC card get to the read-only state, which is why I moved to the USB drive).Yes, ubifs is quicker the jffs2 (apparently) but the older U-boots don't have any ubiload command.

Everything else is on an external USB drive, but there are/were/could be problems trying to boot from USB (which is through a hub), so at the moment I have /boot left on the SDHC card.
But that seems a bit of a waste of a 16GB SDHC card (which also juts out from the box), so if I were able to use the NAND instead (with ubifs - not jffs2) I'd free up the SDHC card and make some use of the internal NAND.

I don't suppose anyone has any details on how to create/setup a file-system on NAND?

In particular, how to create (say) 100MB and 400MB ubifs partitions, put /boot onto the former and leave the latter as and empty but mountable (writeable) file-system?

It hasn't shown an update for a while.   It hasn't yet got any of the 2.6.39.x series.

What is that configuration.
Where have you mounted the drive? ((Evidence?)
You mention network access - what sort of network access?
What happens if you access the (mounted) file-system directly from the Plug?

It hasn't made it to the mirror yet.

Ah - you said a USB drive.  IIRC a MyBook is weird and has 4 partitions (and a specific driver for Windows?).  So you need to do multiple LUN scanning.
e.g.
http://www.linuxquestions.org/questions/linux-hardware-18/problems-with-wd-my-book-1tb-external-drive-564130/
http://www.linuxforums.org/forum/red-hat-fedora-linux/104572-mount-wd-my-book-usb-drive.html

Assuming you have mail setup to work...

Run a cron job everty 5 mins that:

Checks the process is running. 
If not:
   if there is no NO_PROCESS file, send a mail
   touch a NO_PROCESS file
if it is there
   delete any NO_PROCESS file.

Part of the spec for SDXC is that the file-system has to be xfat(?), which is a proprietary Microsoft file-system.  There are, I believe, some digital camera that support it.