Recently my old fusion drive (1T HHD + 128G SSD) on my 2012 late Mac mini might have issues. The OS is very slow and has some read errors when I tried to use CCC to clone it. Anyways, I managed to copy some necessary files to another external drive and am trying to replace this fusion drive with one 2TB SSD and do a clean install. Do I need to split the fusion drive before I take them out of Macmini if I want to use them as two separate external drives later or I could just remove them without the split and format them later as two external drives? Thanks.
You can remove them without any fuss or erase. Fusion is a software layer, not something that changes on the hardware side.
You can remove and erase / format them in any order you like. If you haven’t formatted then yet, maybe try that unless you’re sure you want an SSD even if they work, but I’m guessing you know they still work since you plan on reusing them.
I had a 2012 Mini with a Fusion drive, and I swapped out the SSD for much larger SSD. (I didn't have the courage to remove the mechanical drive, which requires 'extra steps'.
As soon as you remove one of the drives , then that breaks the Fusion scheme.
When I rebooted, the mechanical drive was reported as an 'Unknown Format', and needed to be partitioned. (Given its age, I only used it for temporary storage of things like installers that I could always download again.)