Problem erasing MacBook drive while booted from external drive

Hello,

I'm booted on a MacBook Pro from an external drive, and I want to erase the drive inside the MacBook using Disk Utility from the external. For some reason or another, it won't let me do this.

When I click on the Erase tab (whether clicked on the disk itself or the volume "Macintosh HD") and try to erase it, it returns with the error:

Disk Erase failed
Disk Erase failed with the error:

Could not unmount disk.

When I try to install an image onto the drive, I get a similar error, except it says "Operation not permitted."

When I tried it while booted with an OS X DVD, it worked fine.

As far as I know, I should be able to do this while booted from the external HD.

Any solutions to this problem?

Thanks,
Jason

 

Solved - I had launched Disk Utility from the internal drive using Spotlight instead of from the External Drive.