Cloning a bootable drive?

HI

A year ago the internal hard drive of my iMac (2011) gave up. i was able to recover the situation by creating a bootable external hard drive which uses macOS High Sierra 10.13.6.

I then used carbon copy to copy all my files across from the internal hard drive to the external one. I then set the iMac to start by using the external drive rather than the internal one. This has all worked really well.

I now want to know if it is possible to create a 100% copy of this bootable (source) drive onto a second separate (destination) hard drive? When I put both the source and destination drives into disk utility I only get a spinning wheel saying "looking for drives". I have therefore aborted after several minutes.

Can anyone advise as to how to do this?

Also, If it is possible to create the clone bootable drive, will I then be able to put the done drive into my MacBook pro (also 2011) and then run the MacBook from the drive rather than using the Macbook's internal hard drive?

Thanks