Trying to re-create fusion drive

I was successfully using a fusion drive on my Mac mini 2012 for some years with a mechanical 1tb internal hard drive plus an externally mounted SSD drive from a defunct 128gb Mac Book Air. After unsuccessfully replacing the original 1tb hard drive with a 2tb hard drive, I put the original 1tb hard drive back in the Mac mini. Now I cannot get the fusion drive to work again again. I am running all the Terminal commands I originally set the Fusion drive up with, but Terminal continuously says the original mechanical 1tb hard drive cannot be unmounted. Please help. I am at my wits end.