Having gradually built up my media collection to near the capacity of my 16TB external HDD, I've reached the point where I'll probably need to build a RAID array to keep the collection in one place. Assuming the RAID array will be at least 32TB, I have a few questions:

1. From what I've read RAID arrays can help mitigate the risk of individual drives failing if extra space is allotted on the hard drives. Assuming a total capacity of 32TB, how much of that space would be reserved by the RAID array for data loss prevention?

2. Is there a certain type of hard drive I would have to use? Aside from my 16TB drive, I also have two 2 8TB drives that I'd ideally like to be able to re-use in the RAID array, but have left them in their enclosures for the time being.

3. If the hard drives in the array have different transfer speeds, does the array as a whole default to the slowest one?

4. Whether the hard drives I already have are compatible or not, what RAID enclosure and hard drives would you recommend?