I inserted 4 different HDDs into the MS4UM and they are were accessible like normal.
I then insert the original 4 HDDs and proceeded to format each one (the long time consuming formatting).
Two 4TB HDD - Look good
Two 2TB HDD - Look good, but then
One of the 4TB HDD which yesterday showed:
Volume with a name, letter, 3725.90 GB NFTS Healthy (Primary Partition)
is now showing:
Volume with a NO name, NO letter, 2048.00 GB NFTS Healthy (GPT Protective Partition) + 1678.02 Unallocated
Is this HDD toast?
Is this HDD failing what caused the complete failure of all 4 HDD originally.