Error 0x80070002 Using Windows Backup

I am using Vista Home Premium SP1. I have been using the backup program
found in "backup and restore center" to backup to an external drive called
"MyBook" successfully. For no reason that I can recognize, backup attempts
have started getting me the message "The system cannot find the file
specified ... (0x80070002). ... MyBook (G. This happens whether I try a
partial or a full backup.

Is the file which has become corrupted on my computer, or is it out on
MyBook?

What do you advise me to do?

Thanks.

Spencer
Please respond only to group.