I’ve a late 2015 27” iMac with 1TB Fusion Drive. Usually I used to be utilizing my macOS via an SSD linked to the USB and I used to be sooo completely satisfied concerning the efficiency of my pc. Someday it restarted itself whereas in sleep mode and… by no means be capable to begin once more;
Signs:
Apple emblem seems once I join my iOS containing exterior SSD, however the progress bar by no means fills up (it stays clean endlessly)
Query mark seems once I don’t join my exterior SSD with iOS.
Command+R doesn’t work and OPT+Command+R don’t proceed after some level throughout begin up
Apple diagnostics say my pc is okay
I resetted my NVRAM and tried all potential startup key combos together with Command+S (I can’t attain to Terminal (some bizarre knowledge log seems and no place to write down a command)
Apple licensed service can’t do something since my pc is taken into account as classic

I talked to the apple skilled, and he mentioned likely my HDD is useless (it’s silent now and I don’t hear it anymore).

Do above signs root from the useless HDD?, even all my iOS knowledge is in my exterior SSD?

Will changing my inner HDD to a brand new inner SSD clear up the issue?

I can’t cut up my Fusion Drive since I can’t attain to the Terminal app, will that be an issue?

What signs do you suppose I’ll have proper after changing my HDD with a brand new Sata3 SSD?

I like my pc and need it again, because of all prematurely