One of the Pies has failed. It failed when I was trying to load it up, and when I looked at it, I noticed that a capacitor, which I think is C3, it's the big one next to the power socket, is loose. It isn't soldered in. I could probably fix that myself, but I think I'll send it back to RS for a replacement. The Pi foundation will probably want to know about such problems; they seem to be very keen on quality.
If something goes wrong with a Pi, all I have to do is take the SD card out of the Pi, and put it in another Pi. But what if the SD card goes bad? I've had that happen with a couple of SD cards. I'll be backing up the data, of course, but I'd still have to reload and reconfigure the replacement card, which can take hours, and involves remembering what I have to do.
So I'm doing a backup of each pi. I'm doing an image backup of the 8gb card, referred to as /dev/mmcblk0, to a file on another server. So first, I share a directory on that server which I'm calling "pibackups", then I mount it on the Pi at /root/backup. Then I do
dd if=/dev/mmcblk0 of=/root/backup/robotarm.img
That should make me an 8gb image of the SD card, onto the other server, ready for imaging back if necessary.