Well, I've just stopped by to say thanks and add a little tip for those that may be trying to create an USB bootable MAC OS Install Disk.
I happen to have an ancient Core Duo MacBook, which is 32-bit only, therefore stuck in Snow Leopard. I had formatted it and replaced Mac OS with Linux, but my wife couldn't get used to it, so I needed to at least install Mac OS back in order to see what I do next (probably install rEFInt in order to try to boot Windows 10 32-bit on this machine). Using the Recovery option in Disk Utility I could never manage to make a bootable disk, but CCC also complained that my USB disk would not be bootable, even when I formatted it in Disk Utility using the "GUID partition" option. So in the end I turned to the diskutil command line utility, and I could finally create the proper partition structure in the USB disk, with a command like the following:
sudo diskutil partitionDisk /dev/disk3 1 GPT HFS+ USB16G 16G
Notice that /dev/disk3 is the device name for the USB drive (you can find this out by using the "diskutil list" command). 1 is the number of partitions, GPT is the partition scheme, HFS+ is the file system type, "USB16G" is the volume name (you can put anything you want) and 16G is the USB drive size (16 gigabytes in this case).
You have to unmount or eject the drive before you perform this command, and obviously you should be extra careful not to wipe your hard drive by mistake (though there's a risk only if you have more than one HDD connected, since you can't unmount your boot drive).
Hope it helps someone - it took me a while to find a solution!