How do I transfer Boot disk from External HDD to newly inserted SSD?
It looks like Disk Utility is being extra cautious and not allowing you to unmount the boot drive to copy it. Assuming your copy of Disk Utility is running from the OSX external drive, you will have to power down and try booting into Recovery.

Normally, you press Command R to enter recovery but I haven't tested with no recovery partition on the internal drive, so you might instead boot holding option to see if the OS can find a recovery HD on the external drive.

If you partitioned both drives and have no viable recovery, you will have to use internet recovery or otherwise get a bootable OS that isn't the one you are attempting to copy.

Worst case, return the bootable drive to the inside of the Mac and run an OS installer to re-install the OS onto the SSD. That will make a proper Recovery HD partition that will allow you to boot and run Disk Utility without needing a bootable OS separate than the two drives you want to copy.