I think justperry in the post above has found the key issue.
Try this:
1. Hook up an external boot source to the Mac and have it powered-up and ready to go
2. Shut down the iMac completely and let it sit for 10 seconds (leave the external device on)
3. Press the power-on button
4. As soon as you hear the startup sound, hold down the option key and KEEP HOLDING IT DOWN
5. In a few moments, the startup manager should appear. It will show you all the bootable volumes that the Mac has found, both internally and externally
6. Do you see the presence of your external booting source?
IF YES - select it with the tab key or mouse pointer, then hit the enter key
IF NO - I'll guess that _something_ is wrong with the OS installation on that volume, and it lacks some critical file necessary to boot the Mac. Or there might be a hardware or connection problem.
Other thought - the Intel SSD may begin the boot process so quickly, that you can't "intervene" to switch-boot from another source.
Seems to me that in the early days of Mac, having a disc installed in the CD drive (they were just CD drives back then, and they could hold an entire system!) would "give it priority" in the boot process -- that is to say, the Mac would "look there first" for a bootable volume, and if one was found, would always try to boot from it _first_, before looking elsewhere.
Not sure if this carries over to present-day. But if something like that still applies, the presence of the Intel drive would be "a substitute" for the DVD drive. And the Mac, finding a bootable volume present there, would "grab it" as the boot volume first, giving you no chance to switch it to another source.
Again, I think the exception is the routine above to invoke the startup manager first.
