After filling up the 64MB drive with pictures, leaving about 150kb free, I backed up using the PC utility. Once again, it did not warn me or tell me the drive was full, but formatted and renamed it PKBACK# 001, then dropped the zip file onto it.
Next, I filled up the drive again, leaving 4MB free, and backed up again. It added the backup file, leaving 2.7MB free. Then I backed up again, and it erased the disk and put the new backup on the drive by itself.
However, I wasn't quite accurate when I said it didn't warn me - I just didn't recognize it as such. A window popped up with a heading of "DynaZIP ZIP Request", like this:
I'll try to remember if I see that again to click on Cancel if I don't want to erase my drive.

