The WD drives were formatted ntfs, and so a 900GB file wasn't a problem. What was a problem was that, on both drives, with Ubuntu 11.4, the latest version of Truecrypt, Gnome instead of that Unity crap, I could not consistently copy a 170GB file to the Truecrypt container--it kept hanging.
The simple and reliable solution was to format the WD drives ext2, trashing the WD software that I couldn't imagine ever using, and then to do my backups to /tmp as .tgz.cpt, i.e., as gzipped tar encrypted using ccrypt. Then I could reliably copy the file to the WD drive.
BTW, with NTFS on Ubuntu, even with ntfstools installed, I never could change the label of an NTFS volume. But it was a three second job on a Windows 7 machine.
So I temporarily went back to Truecrypt, but returned to ccrypt begging forgiveness. A zombie process told me "It's too late to say you're sorry. How would I know? Why would I care?"