Really appreciate your words and thank you for taking the time to share with me the pro-tips and an outstanding explanation.

I have one question: One of my other disks has very sensible data so I am currently buying another 6TB WD RED disk for my nas to replace the failed one and I plan to use the new one in a RAID array to replicate the important disk. Would you recommend another model of HDD? What type of Raid would you recommment to use in this scenario?

Many thanks

There are performance impacts to RAID as well. Read speeds generally are better with RAID than they would have been on individual disks. You get to read from all the disks rather than just one so you basically take the read speed of one disk and multiply it up by the number of disks (e.g. if you were getting 175MB/s read speed out of an individual disk you could get about 350MB/s out of a two drive RAID1 or 875MB/s out of a five drive RAID5 or 6). That will very quickly get limited by other factors though (i.e. that NAS has 10GbE ports but if you are then running through a 1GbE switch you'll saturate the network at about 125MB/s anyway). Write speeds take a hit under parity RAIDs (5 or 6 in this discussion). If this is a business environment and you have a lot of people writing to all the volumes concurrently then they probably have better write performance under your current setup than with RAID 5 or 6. RAID 5 makes 4 I/O operations per write and RAID 6 makes 6 so you take your total write capacity and divide by 4 or 6 respectively. If you are mostly doing reads, or if writes are spread out and you don't have many people writing at the same time, or if write aren't performance sensitive like backups happening in the background, then that won't affect you.

Personally I'd go RAID 6 if it would provide enough capacity for the data you store. Other people on this forum would disagree strongly with me and go RAID 5. Your call as to balancing redundancy vs performance vs capacity (the latter of which ultimately comes down to price).

Just another reminder, regardless of RAID level you'll still want to backup your data somewhere other than the NAS for all the reasons I called out before. Look into a cloud storage solution and setup hybrid backup sync to automatically push your data up to the cloud (at least the very sensitive data). That way in the event you lose your whole array (or entire NAS in the case of fire or theft or such) you can still recover.