For the benefit of other newbies: Although the tutorial does say that...
The Debian installer doesn't currently support installations to on-board flash storage, but you can use it to install to USB, SD or eSATA. In order to proceed, you will therefore need either a USB stick (or disk), an SD card or an external disk with an eSATA port.
... I was confused because it said further down that it's possible to download the installer from a TFTP server.
But that was really the installer
: Debian itself cannot be installed directly on the NAND flash memory, so you really need to first plug a USB stick into the Plug before downloading and running the installer.
With the USB stick plugged in, you'll get to this better-looking screen:
+------------------------¦ [!!] Partition disks +-------------------------+
¦ The installer can guide you through partitioning a disk (using ¦
¦ different standard schemes) or, if you prefer, you can do it ¦
¦ manually. With guided partitioning you will still have a chance later ¦
¦ to review and customise the results. ¦
¦ If you choose guided partitioning for an entire disk, you will next ¦
¦ be asked which disk should be used. ¦
¦ Partitioning method: ¦
¦ Guided - use entire disk ¦
¦ Guided - use entire disk and set up LVM ¦
¦ Guided - use entire disk and set up encrypted LVM ¦
¦ Manual ¦
¦ <Go Back>
However, Martin's tutorial ends with:
Your plug computer is now ready to boot Debian from USB, SD or eSATA and it will automatically do so whenever you turn on the plug computer
Since the Plug has a 512MB NAND flash memory, it'd be too bad not to use it and be required to keep a USB stick plugged permanently to run Debian while this wasn't necessary to run Ubuntu.
Thankfully, Matthias Blaicher's tutorial
explains how to copy Debian from the USB stick to the NAND flash memory: In the "Partition disks" where I used to be stuck, I chose the default option "Guided - use entire disk", and the Debian installer detected the USB stick as "SCSI1 (0,0,0) (sda) - 3.9 GB Kingston DT 101 G2", and completed the install onto the stick.
I'll report back once the installer is done copying Debian to the stick.