Move Two Drives were mirrored to NTFS

Question

I have two 4TB Drives that were mirrored (Storage pool I think) used as a D:/Data silo. I did a third (External) drive backup and physically replaced them with four 8TB drives in a Raid 10 (Storage pool) and configured a 16 TB silo as D:/Data. I have moved the two 4 TB drives in two USB 3.0 external trays and I would like to reform them into individual NTFS 4TB drives to use as even and odd drives in a BU schema in a smaller PC that belongs to a friend. The 4TB drives are low hours and they are enterprise NAS class drives so I have little concern about their reliability for the future use.