Chosen Solution

I have a question. I am currently running Catalina on my late 2013 27 inch iMac. My iMac has a fusion drive which has a 3TB HDD and a 128 PCle SSD. I bought a SSD that will replace my mechanical 3TB HHD, as well as a larger PCle SSD that will replace the 128GB SSD on the mother board. I was told I don’t need to do anything but replace both of these and I will be able to use either of the new ones for what ever I want and that they will both show up in disk utility and I can format them each separately and use either one or even use them both as separate boot drives with macOS on one and another macOS on another one. When I read this post I noticed that someone asked the question about splitting the fusion drive in order to be able to access both the new drives seperatly when the new ones are installed. Will I need to worry about this or am I good to remove them both and replace them both with new ones and be able to access both of them to use separately? I was planning to clone my entire iMac to one of the new SSDs before I open up the iMac. Then replace the internal hard drives. I am not trying to create a fusion drive inside the imac when I replace both hard drives. I am only trying to replace them both and use them separately inside. My only concern is will I be able to access the PCle SSD once I install it? I know I will not have any problems accessing the regular SSD that replaces the mechanical HDD.

Yes you can split the fusion drive. You can even do it before you replace the drivees. See this article: https://www.lifewire.com/split-fusion-dr….

I would suggest to do the following. A Time Machine backup (or 2), Maybe one to a network storage and one to a local attached usb/firewire disk.Then replace the 2 disks.Do a fresh install of Catalina, you will get an option to choose one of the new disks.During install use to option to Recover from a Time Machine backup and use the one on the local attached disk. Some more detail of the fusion drive. If you go into terminal and type: % diskutil list You will see something like below. it shows your current ssd and hhd, /dev/disk0 and /dev/disk1 in the list below. After that you see another disk which is: /dev/disk2 (synthesized) which is the fusion drive. That is where your information is stored. When you do the fresh install, the install proces will create the required disk layout with the recovery partition etc. Diskutil list output: /dev/disk0 (internal, physical):    #:                       TYPE NAME                    SIZE       IDENTIFIER    0:      GUID_partition_scheme                        *121.3 GB   disk0    1:                        EFI EFI                     209.7 MB   disk0s1    2:                 Apple_APFS Container disk2         121.1 GB   disk0s2 /dev/disk1 (internal, physical):    #:                       TYPE NAME                    SIZE       IDENTIFIER    0:      GUID_partition_scheme                        *1.0 TB     disk1    1:                        EFI EFI                     209.7 MB   disk1s1    2:                 Apple_APFS Container disk2         1000.0 GB  disk1s2 /dev/disk2 (synthesized):    #:                       TYPE NAME                    SIZE       IDENTIFIER    0:      APFS Container Scheme -                      +1.1 TB     disk2                                  Physical Stores disk0s2, disk1s2    1:                APFS Volume imac01 - Data          761.6 GB   disk2s1    2:                APFS Volume Preboot                 81.4 MB    disk2s2    3:                APFS Volume Recovery                529.0 MB   disk2s3    4:                APFS Volume VM                      4.3 GB     disk2s4    5:                APFS Volume imac01                 11.3 GB    disk2s5