Thanks for responding, McButton. As far as I can determine, the reason I ended up trapped in the black screen is that I had used the following command from the wiki, sudo set_gbb_flags.sh 0x489, which sets the default boot to legacy BIOS and the delay time to one second. I guess for most people this is not a problem, but if the legacy BIOS fails to work (as it did in my case), there is no obvious recovery. With only a one-second delay, there did not seem to be a way to intervene. Perhaps Esc-F3 would have worked if I had known about it--I'll have to make a record of that. The way I finally escaped was to hold down Ctrl-D *before* I powered on the machine, which took me to the Chrome screen and gave me an opportunity to reset the gbb_flags.
I don't think I was clear enough on the problem I had with making a ChromeOS backup. The problem is not with creating a restore USB stick--I did that no problem and that is how I restored my machine back to factory specs. The problem is with creating a copy of the ChromeOS from the Chromebox hard disk to a USB stick or SD card in order to be able to boot the operating system from one of those external media. I followed the instructions exactly as described in the wiki,
http://wiki.xbmc.org/index.php?title=ASU...SB.2FSD.29 . When I execute step 3,
sudo chromeos-install --skip_src_removable --skip_dst_removable --skip_vblock --dst /dev/sdb,
It looks like the copying process starts just fine. I get a number of messages indicating that partitions are being installed to /dev/sdb, starting with partition 12 and counting down. There is one warning after the installation of partition 11:
WARNING; DOING A SLOW ddOPERATION. PLEASE FIX.
but there is no indication that this is a fatal error, as the process continues. I get a message
16384 + 0 records in
16384 + 0 records out
8388608 bytes (8.4MB) copied
after the warning.
After messages indicating installing partitions 10, 9, and 8, there is a message
8 + 0 records in
8+ 0 records out
16777216 bytes (17MB) copied
and then further messages about installing partitions 7, 6, and 5.
It is after the message "Installing partition 5 to /dev/sdb" that there is a long period of no reported activity, the system reboots, and I find that Ctrl-U just causes a beeping noise, and Ctrl-L followed by ESC and choosing the USB stick or SD card does not run the OS from these media. Instead, a message about booting from the hard disk appears. That is what leads me to think that the process of copying the OS to the external media failed at partition 5.
I've followed directions to that point scrupulously, including flashing the BIOS with John Lewis's version that is supposed to enable Ctrl-U and setting the developer boot flags as described in step 1.4 in the wiki. Because the wiki says, "It is highly recommended that you create a backup/USB copy of ChromeOS," I am reluctant to proceed past step 1.5 until I successfully create the bootable USB stick or SD card. Again, suggestions on completing this step successfully would be appreciated.