Can I replace my 1 TB Fusion Drive HDD with a new SSD without splitting the Fusion Drive first ? (because splitting is not possible)

Then the wise thing to do is to replace the computer!! Don't spend $$$ on 11 year old antiquated technology. It simply does not make sense. If you cannot afford a new Mac you have a lot of alternatives. First is:

• Apple Refurbished: Zero downside as they are identical to new, have the same warranty as new, are eligible for AppleCare and can save hundreds of $$$. You can find the Refurbished Store at apple.com then scroll to the bottom of the page and select the Refurbished store.
• Don't buy a new iMac: You are not "locked" into an iMac. Buy a Mac mini which is much more powerful than your 2015 and does not have any moving parts other than fans. They all come with SSDs, get one with at least 1TB SSD and 16GB of RAM and you will have a nice computer. Then buy any display you want to go with it, display start at <$100 so cost is not a deterrent.
• Buy a used Mac from a reputable reseller: Navigate to Macsales.com which is a trusted reseller of used Macs. They are fully checked out and are ready for year of service. You can save a LOT of money this way. Never ever buy a used Mac from:

EBay

Craigslist

Nextdoor

Marketplace

Local PC shop

An acquaintance or friend

Why, because these machines are NOT set up correctly in most cases per Apple's advice in What to do before you sell, give away, or trade in your Mac which means they NEVER will work correctly. Also, in many cases you will have the same issue you had with the logic board you bought, it just won't work!

onpoy wrote:

I cannot split my Fusion Drive since I cannot reach to the Terminal app, will that be a problem?

If the hard drive that is part of the Fusion Drive is dead – or you have pulled it out and replaced it with a SSD – you have essentially split the Fusion Drive (as in, "bye bye, data").

I don't know if there is something left on the SSD part of the original Fusion Drive that is confusing the Mac. If the repair shop had removed the original SSD at the same time that they replaced the dead HDD with a SATA SSD, the result might have been better. (There would have been no "blade" SSD with part of a volume on it, to maybe trick the Mac into thinking that it still had a Fusion Drive.)

But it sounds like there may be something wrong beyond that, if the Mac is willing to start booting from an external drive, but then hangs while booting from the external drive.

P.S.: The Mac operating system is macOS, not iOS. iOS is the name of the operating system on the iPhone.

Hi,

I agree with Apple Expert, your iMac HDD is dead. Old fusion drive iMac is separate installed as two drives, HDD and SSD. You could replace internal dead HDD and SSD with your external SSD, though it's quite hard work.

It's quite easy to select startup drive with press and hold the Option key as you press the power button to start up your Mac and select external SSD as startup drive which could works.

First never ever take a Mac to a PC guy, they are not correctly trained. Please take your computer to an Apple Authorized Service Provider in your area. If you don't know of any then click AppleCare Contact Info.

Next, I know you would like to fix the computer however please remember it is 11 years old now and is not only technically obsolete it is also classed as vintage by Apple meaning parts are not available. Finally it cannot run current versions of Mac OS.

I would strongly recommend spending your money on its replacement. A Mac mini will run circles around a 2015 iMac that is running perfectly. All you need is the MM and a display and you are set.