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.

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