[G4] Erasing a Hard Drive

[Ron Michael]

I'm trying to erase/reformat a Hitatchi 80gig hard drive in my mirror 
door G4, 1.25mhz, which is not my primary system drive. I had formatted 
it MS-DOS with the intention of taking it out and making it an external 
drive for my laptop. I changed my mind. Now I want to reformat it with 
Mac HFS. I tried using Disk Utility Erase function and it won't let me. 
Says it can't unmount the drive. Any suggestions? Any other program 
that might handle this for me. I've got Disk Warrior and the drive 
won't show up there either.
Thanks.
Ron in Memphis