Upgrading Fusion Drive to SSD, With added RAM - How and What Parts

The blade SSD is a cache drive, your OS is on you HDD. Sadly, the 24GB SSD is too small for a boot drive.

I would replace the blade SSD with something larger making it the boot drive holding your OS & apps. I would leave the HDD alone using it for your data drive. Basically setting up a dual drive config.

Here’s a good reference on Apples custom SSD’s The Ultimate Guide to Apple’s Proprietary SSDs you can go with an OWC drive besides the custom Apple units.

First thing is to make a clean backup of your system to be safe!

You'll need to break the Fusion Drive set follow this guide: Split Your Fusion Drive Apart

Then install your replacement blade SSD following this guide: iMac Intel 21,5" Retina 4K Display Blade SSD austauschen

Using Disk Utility format the new drive GUID Journaled file system and make sure you label the drive so you can tell the difference between the drives.

Now restart your system pressing Command (⌘)-R to enter into macOS Recovery About macOS Recovery Select your new drive to install a fresh copy of the OS your system is currently using.

At the completion of the OS install you will be asked if you have any user data to transfer to your new drive. You will want to move the user accounts and apps over.

Reference: Mac startup key combinations