RAID 1 on extended volumes

Sigh.

I also want to have some type of redundancy in place...If it can be done what would happen if I have a hard drive fails?

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Do you need redundancy? Do you know what that means? Most people come in here thinking redundancy means backup, and it's not. Having redundancy and having a backup are solutions for two different issues. Redundancy allows your media server to stay up and online in the event of a drive failure. You aren't Amazon. You aren't a huge business that NEEDS that info guaranteed to be online 99.999% of the time. I can almost assure you what you want is a backup. Which isn't solved by any form of RAID.

To answer your question, we'd need to know what version of NAS and/or media server you are running. I'm not up to speed on them, but if you wanted to run a RAID 5 setup and your NAS is an older/basic model it might not support that. What I can say is because you don't need 99.999% uptime is to ignore all RAID setups. Buy whatever new drives you want, and find some way to do a real backup in the event of a drive failure. This means the data is backed up via USB or expansion drive which is physically removed from the system. You'll need to remember to plug it back in every day, week, month, etc to back up the new data. This way if a drive dies you can replace the drive and then attach the backup back into the NAS and copy the data over. This is what you (probably) want.

Backing up 4x6TB is a rather big ask for a home user. Tape drive, safe storage for tapes and such. That is in my opinion a waste for a home user. A RAID redundancy should work as a decent safeguard for most people. It's not a backup in the sense that you can restore older versions of selected files, but it is a good safeguard as losing two disks at the same time is fairly unlikely.

RAID won't help when you get a crypto virus and it encrypts everything. Won't help when you get a lighting strike and the drives gets zapped. Won't help if your kid/other clueless person presses the wrong button and deletes something. Or if you are moving things around and accidentally press the wrong button yourself. Backups might take time, but they are the only way to prevent ANY of the above from deleting data. RAID 1, 5, 6, 10, etc would all have data loss.

I wouldn't do tape drives. USB 3.0 or some sort of external dock is what's needed. Plug in an external drive and run the software. It can check the folders it's been told to scan and then copy over the new data it finds since the last time it was run. Remove and put back where ever you store your backups. It takes time to setup, but it's the only way to prevent data loss.

Edit: I bought the HAF XM. I bought it for the two external drive bays. I take an old SATA drive and pop it in. Copy anything I want to save, and then pull the drive back out. The drive bays are hot swap and I can back up anything anytime I want. (This doesn't work as well as I want due to the horrible drive cage system Coolermaster uses, but this idea works.)

First: RAID is not a backup. RAID is for eliminating downtime. If your company will lose a million dollars a day in business if the file server goes down, you want that file server on RAID so a single HDD failure does not bring it down and cost you a million dollars in the time it'll take to restore a backup. i.e. A file server on RAID will keep functioning despite a HDD failure.

If you were hoping to use RAID as a backup, that doesn't work. If you accidentally delete or overwrite a file, or malware encrypts it, the redundant copy or parity data will also immediately be deleted, overwritten, or encrypted, and you will lose the original file. Most crucially, this means that even if you're running RAID, you still need to make backups.

So you have to ask yourself what you're trying to accomplish. If you need this media server to be up 24/7/365 with no downtime (e.g. your kids will kill you if it goes down), then you want RAID. But the vast majority of people don't need this type of reliability from a media server. If some downtime is acceptable, then you don't need RAID. Just use a single drive (or combine multiple drives into a JBOD), and be sure to make regular offline backups.

If you really do need RAID for this application, then you've got the math wrong.

• 
  ■RAID 0 gives you the full storage space of both (all) drives, at reduced reliability. If you have two 6 TB drives in RAID 0, you have 12 TB of storage.
  ■RAID 1 gives you n/2 times the storage space. If you have two 6 TB drives in RAID 1, you get (2/2)x6 = 6 TB of storage.
  ■RAID 10 also gives you n/2 times the storage space. If you have four 6 TB drives in RAID 10, you get (4/2)x6 = 12 TB of storage.
  ■RAID 5 gives you n-1 times the storage space. If you have four 6 TB drives in RAID 5, you get (4-1)x6 = 18 TB of storage. Getting 12 TB of storage from RAID 5 only requires three 6TB drives. However, the chance of a second drive failure during a rebuild are high enough that RAID 5 is not recommended for multi-terabyte volumes. Use RAID 10 or RAID 6 (n-2) instead.

Regardless of which RAID you use, you still need a backup. So the final numbers for 12 TB storage using 6 TB drives end up:

RAID 10 = 4 drives for RAID + 2 drives for backup = 6 drives total
RAID 5 = 3 drives for RAID + 2 drives for backup = 5 drives
No RAID = 2 drives for server + 2 drives for backup = 4 drives

If a drive fails in a RAID array, the RAID hardware (or software for software RAID) informs you of the failure. In RAID 0 your data is just gone, and you have to restore from backups. In the other RAID versions, you remove the failed drive and replace it with an identical new drive. The RAID hardware/software then rebuilds the RAID array. In RAID 1 and 10, it copies the missing data from the good drive to the new drive. In RAID 5 and 6, it takes the data on the good drives, re-calculates the parity info which was on the failed drive, and writes that onto the new drive. Either way, for 12 TB of data you're looking at over a day of rebuild time. Except for RAID 10 or 6, a second HDD failure during that rebuild stage will destroy the array losing all the data, and you'll have to restore from backup.

Time to restore from a backup is about the same as a rebuild. So the only benefit RAID gives you is that the data will be available during a rebuild. If this isn't important, then you don't need RAID, just a backup. Given the un-changing nature of Plex media files, you may want to investigate into burning backups to dual-layer blu-ray discs (50 GB each). The price for a 50-pack is down to around $80, which works out to $32 per TB. A 6 TB HDD is around $150, or $25 per TB. So about the same, except the blu-rays won't lose data if you accidentally drop them, and can't be accidentally erased (I once copied a corrupt file over its backup, instead of the other way around).

It frustrates me that people think they need everything a business uses. Xeon CPUs? ECC memory? Windows pro over home? RAID setups? Home users need none of this. Maybe the Xeon if for some reason it's cheaper than the i5/i7 it's based on and it can drop into a normal board making the system cheaper. But there is nothing found in any of this business class stuff that will help a home user.

So you already have a copy on your PC/HTPC? If the NAS truly does store a second copy, that's your backup. If it stores your only copy and you just stream over the network, you are still only a single drive failure away from losing everything. RAID will protect against some forms of drive failure, but not all. RAID 6 and 10 require extra drives. Might as well buy the extra drives and use them only sometimes as a true backup.

A true external backup is what I personally feel is best. If you can go through the hassle of setting up RAID, you can go through the hassle of backing up the data for real. Not everyone agrees of course and it's your call as to what to do. But why protect against only some forms of drive failure? Why not all forms?

Edit:

It frustrates me when you come to a place looking for help and you get asinine answers like this. If a question frustrates you so much there is always that option of saying nothing moving on

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Because sometimes people hear something, and it's not true. RAID will protect against some forms of drive loss, not all. Sometimes people read about something, or hear about something and mistaking think that's what they need/want. A home users use case isn't the same as a business's. Why use what you don't need. It's not an "asinine" answer. It's a correct answer. Sorry to give you the right advice. No need to be rude simply because it doesn't match what you've heard.

https://www.merriam-webster.com/dictionary/asinine

extremely or utterly foolish or silly; of, relating to, or resembling an ass extremely or utterly foolish or silly; of, relating to, or resembling an ass.

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Yes, '18TB'.

I have a RAID 5 in my NAS box.
4 x 4TB drives.
12TB (actually 10.6TB) volume space.

Why the RAID 5 in that NAS box?
Simply an experiment. Absolutely not needed for any redundancy or backup.
Backups in my realm are actual backups.
http://www.tomshardware.com/forum/id-3383768/backup-situation-home.html