USB drive External can not be formatted

[mashinhead]

I bought a new hd for my macbook when i first got it. I had the original drive which i didn't use for a while, and recently got a housing for it to be external. It connects via USB and powers via usb no external adapter. It mounts as the original 'macintosh hd' because it has os x installed but when i try to reformatt it in disk utility i always get the same error, drive can not unmount. i can't even repair permissions.

 

[jeremy.king]

You can only repair permissions on the boot drive.

So are the two drives named the same thing? If so, try to rename the external drive and then retry formatting.

 

[Eidorian]

So are the two drives named the same thing? If so, try to rename the external drive and then retry formatting.

OS X handles that chance. I've had two drives named "Macintosh HD" on a single machine. The non-boot drive changed itself to some random string of characters for its name.

Still, have you tried using a Live CD or another computer to format it?