How to reset a Seagate FreeAgent Go Mac External Hard drive to use with a new Mac? I have a 500 GB external hard drive that I want to use with my new Mac (OS X Yosemite). I simply want to clear the external drive of everything that was on my old Mac (I don't want to transfer any files) and start from scratch. How would I go about doing this? Seagate doesn't address this question on their site and they have not responded to my inquiry. 

When I attempt to erase using Disk Utility I get this error message: 


  Disk Erase failed with the error:
  Couldn’t unmount disk.
Boot into the Recovery HD by holding ⌘R on boot and use Disk Utility on there. This way you can guarantee the disk is not being accessed and can be unmounted successfully.