I made a bootable clone of my iMac with mountain lion using Carbon Copy Cloner. This bootable clone opens fine on the old Mac. I tried to open it on my newer Mac with High Sierra, but when I click on the drive I just get a circle with a bar through it and it doesn't load. Is this because there is a different OS? The reason I want to clone the old hard drive is in case that Mac dies and I would like to be able to access the old data and run the older OS as I have software on it the won't work on the newer OS. Is there a way to do clone that drive so that it loads with mountain lion running on my newer Mac?
To be absolutely certain we'd need to know precisely which Mac your newer one is, but the general rule is:-
You cannot boot a Mac to an OS older than itself.
The drivers for the yet-to-be-invented hardware simply did not exist in the older OS, so it will not boot from it.
Solutions may be
1. Spare older hardware in case of failure of the original
2. A Virtual Machine made from the older OS - Parallels or VMWare etc.