Thanks for your reply, Choong. It was sufficient to prompt me to discover how to "restore the system using the Recovery Partition", and if anybody else gets into this kind of pickle, here's how I got out of it.
Source: Intel® Compute Stick STCK1A8LFC - User Guide
Following these instructions, and choosing the top most 'Recovery' item in the boot menu, the ICS booted into Ubuntu with only an 'Ubuntu Recovery' dialogue with which to interact - including a warning about how "you will lose all your data". Since my data are generally in cloud storage (eg: Dropbox, Evernote, et al), I confirmed the recovery process and clicked on [Continue]. Then...
● Install (OEM mode, for manufacturers only) > Copying files... > Installing system... > Configuring hardware... > etc., &c.
● Fresh installation boot sequence > System Configuration > regular reboot
● Back to where I was when I first used my ICS
So now I'm faced with redoing all the updating, installing, uninstalling, and configuring I've ever done to get back to where I was before the Ubuntu Security Update bricked my ICS. It may be a 'sledgehammer to crack a nut' solution, but it is at least proof that the built-in 'Ubuntu Recovery' process works.
While recovery works, I'm astounded that Ubuntu is soooo lacking in robustness that what appeared to be a bog standard OS update procedure can wreak such devastating havoc. Maybe just presuming a boot drive has oodles of free space works OK for the great majority of Ubuntu systems, but any OS update process that fails to halt with an appropriate 'insufficient disk space' error message as it drives free space down to 0 bytes seems to me to smack of awfully shoddy programming.
"What I ended up doing was to get a Windows ICS and re-install the OS. This new configuration is "comparable" to a standard desktop installation in terms of behaviour and usage."
Do you mean replacing Windows 8.1 with Ubuntu 14.04?
"I found Ubuntu ICS to be very limited in terms of RAM and Storage."
Seems we agree on the inadequacy of the Ubuntu ICS, while drawing different conclusions - on discovering Intel had removed the Ubuntu 'Suspend' command, I wrote:
"Frankly, I've found Ubuntu 14.04 on the ICS (onna quad-core Intel Atom Z3735F processor, July 2014 vintage, 1.33 GHz to 2.16 GHz) to be so utterly lacklustre compared to OS X 10.9 on my old MacBook Pro (onna dual-core Intel Core 2 Duo processor, January 2008 vintage, 2.5 GHz) that the Ubuntu ICS has been relegated to being an emergencies only contingency device - one which adds resilience, but isn't in day to day use. While the Ubuntu ICS can just about do one thing at a time, it is soooo woefully under-resourced in RAM and storage that it is a frustratingly slow and inept computer to use. So it's looking very much like I'll need to spring for a more competent and expensive contemporary Ubuntu PC (eg: System76 Meerkat » https://system76.com/desktops/meerkat ) to get a system that exceeds a 2008 MacBook Pro."
(from 'ICS with Ubuntu: Missing Suspend Function?' » https://communities.intel.com/thread/77227?sr=stream&ru=527589&forceNoRedirect=true )
It's only because my old MacBook Pro is away getting a newly noisy cooling fan quietened down that I turned to using the Ubuntu ICS temporarily. Given Ubuntu's demonstrable failing, I'm beginning to wonder if buying a more expensive Ubuntu PC might not be a case of unwisely throwing good money after bad.
Yes, I replaced Windows 8.1 with Ubuntu 14.04. Although Ubuntu works much better, but it is still different from a desktop implementation in terms of features. For my case, I am using the ICS in a digital signage solution, but the lack of "Wake on Real Time Clock" feature in the BIOS is frustrating, especially when one of the advertised use cases is for digital signage.