I was always too cheap to do RAID, but this was the last time I wanted to lose data from a dying drive. I already had one 5TB drive installed in my NAS, so I wanted to buy two more 5TB drives in order to have 10TB of space with 5TB of redundancy. Meaning that one hard drive can die and I do not lose any data. I used the Synology RAID calculator to determine this. The problem was that I had over 2TB of data already stored on my 5TB drive. In order to do RAID5, you need to reformat from scratch and use a minimum of 3 drives. I had nowhere else to put the data temporarily! Luckily, Synology has something called SHR.
Synology Hybrid RAID (SHR) is like your traditional RAID, but it provides the opportunity to expand volumes without losing data. So what I could do is create the SHR with the two 5TB drives, move the data from the third drive, and then finally include the drive to the SHR volume.
Searching around online, I found a good deal for some 5TB WD Elements Desktop external drives for $130 each. Users have reported that when opening them up, they found Western Digital (WD) RED drives in them. WD RED drives are ideal for storage, specifically NAS devices. But the drives are not cheap, being closer to $200 each. On the other hand, you void the warranty when ripping open these cheaper external drives. My 3TB drive that died was cracked out of an external, and checking the drive warranty with Seagate shows that it cannot be RMA'd as-is. And of course I had thrown away the casing to try and put it back together. Anyways, I decided to take the gamble and purchase the two 5TB WD Elements in hopes of getting lucky.
The Luck (or lack of)
Unfortunately, lady luck left me dry. They were two WD Green drives, which are part of the "power efficient" series. If you use these drives as-is, they will not last very long before they fail. They are very aggressive in terms of parking the head of the drive in order to save power. This causes excessive wear on the drive when using it for storage and constantly accessing it for reads/writes on a NAS. I had to use idle3-tools on linux in order to disable this. Western Digital provides this for Windows users directly, called wdidle3.
After "fixing" the drives for NAS usage, I proceeded with my theory and formatted them into an SHR volume. I then transferred the data from the third drive and then added the drive into the volume with success. It actually took about two days in order to accomplish this. But now I have 9TB of usable storage, which should last me a very long time without having to worry about losing data from a dead drive.
Reusing the shell left behind
Since I took out the drives to use internally, maybe I could still use the WD case for other drives? Well, it isn't possible. Unless you will be replacing it with different 5TB drive, which I wouldn't see the point, it won't work. The 5TB disk size is hard coded into the controller. No matter what size disk you put in it, the OS will always read it as being a 5TB drive. And two things will happen, either the OS will correctly stop and tell you that it cannot format/use the drive or you will lose data. I just ended up throwing mine away.