Thank you so much for this very helpful information. Yes, it's an old computer, exactly nine years old.
Using a different Mac, Carbon Copy Cloner also failed to clone my computer's drive (in target disk mode) onto another drive, saying that "an error occurred while trying to access [computer name]/.symAVx86QSFile, which although I have no idea what that is, it sounds consistent with your suspicion. I've had a succession of hard drives that I use for back up (and for use by Photoshop as a scratch disk) go bad on me in the last year and a half and I just had one of them checked-- it's a damaged file system. Could there be something on my computer that isn't affecting its behavior (it works fine, and until recently used Carbon Copy Cloner without incident) but is creating all this other havoc?
My latest backup disk, with the bad file system, may be completely retrievable; I'll find out in the next two days, but in any case I have dragged everything possible off my computer onto the newest external drive, the one that CCC was unsuccessful with, so I'm not in that much danger of losing data.