In case anyone reads this thread in the future, I found out the answer to my question.
The current DBAN version (1.x) will not work with a USB drive. The beta version (2.0x) will recognize a USB. In fact, when you boot via CD, it gives you a choice of running the program on either your computer's internal hard drive, the USB drive or both.
I used it last week on a USB drive. After running it once, a friend suggested I run it again with some advanced options to "really" erase all the data. After that operation finished, however, Windows no longer recognized that there was even a drive present.
In trying to search for a solution on my own, I found this Microsoft Support topic, "How to use Disk Management to configure basic disks in Windows XP,"
which reinitialized and repartioned the disk. All is now well.