OP:
The LAST THING you want to do (if you put an SSD into it) is to "re-fuse" the two drives.
Just "let each drive be ITS OWN drive". This is simpler and less chance of problems afterwards.
I'm guessing that the platter-based drive is failing, and interfering with the bootup process somehow.
There's another thing you could try.
You said you have ANOTHER Mac, right...?
Which Mac, what year made, what OS is on it...?
What might be do-able:
- Get an external drive
- Use your WORKING MAC to install a copy of the OS onto the external drive
- Now, boot from the external drive (use the WORKING Mac first), and create a basic administrative account (just username and password).
- Take this drive to the problem iMac and see if you can boot it to the finder.
The idea here is that even with a problem internal drive, the iMac -might- be able to boot to the finder from an EXTERNAL bootable drive.
Other than this, perhaps the only way you're gonna get the iMac bootable again is to open it, replace the HDD, and try things that way.