You bought the NAS but not a stack of NAS-rated drives. Here is how to figure out which disks already in your possession are safe to put in an array, what caveats each drive type carries, how to estimate the remaining life of an older drive from its SMART data, and how all of that constrains the RAID layout and capacity you can actually build.
Raid
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NAS Drive Selection: Reusing the Disks You Already Have