Using dd I'm getting a Resource Busy error

Hello, I'm trying to restore an old Hard Drive image OS.dmg to a usb hard drive that I have.

I'm using the command:

dd if=/Users/Shared/OS.dmg of=/dev/disk5 bs=1m

And I'm getting

dd: /dev/disk5: Resource busy

What would cause this error? I'm not sure why it's "busy"

Thanks

Josh

PS.. I get the exact same thing when i try to do it in DiskUtility