Can't open bootable copy of old Imac with Mountain Lion on New iMac with High Sierra
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.