[G4] Can't Erase

[whshaw]

Hello,
I have an eMac using OS 10.4.2.
I have an external WD HD that I use as a backup drive.
I decided today to erase it and start over with automatic backups every 
week. I used the apple utility program. I set it to erase and after 
erasing for a while the program stopped with a note
Volume erase failed with the error- Could not unmount disk.
This is a new one to me. How do I get around it?
Thanks

Wilton