My Synology is a DS918+ and actually has installed three WD Red with each 6 TB of space. The problem is that two of them have a “power-on time” of >40’000 hours. That means, that they have been running since over 4.5 years.

The normal estimated lifespan of NAS HDs is about 3-5 years (it depends on the amount of data written and other factors like temperature). In any case the probability that this two HDs will fail is high.

The new disks

After some research I decided to get the Seagate IronWolf with 8 TB of space. It seems that they are performing better and they are cheaper (~10%) as the WD Red. One negative point could be, that the fail rate is higher. There are many people reporting, that a lot of Seagate are failing already after 3 years. We will see

Requirements

Requirement for the disk replacement is that you have a storage pool on a RAID. In my case I have a Synology Hybrid RAID (SHR) with 1-drive fault tolerance:

Another requirement is that the new disk has at least the same space as the old one or more. Pay attention that there could be some differences between manufacturer. For example 6 TB WD Red or 6 TB Seagate could have some differencies that would fail the replacement.

Replacement

The replacement is the easiest part. Take one HD out and put the new one back in the Synology.

Now go to your Storage Pool and start the repair process as show below:

Depending on the HD size it takes some hours. In my case about 20 hours / disk. During the repair process the most things are working as usual. I found only one problem with the Video Station: all the videos could not be played because of insufficient disk volume size.

Speed test

I did also a simple speed test copying and pasting a video file (~2 GB size) locally. The WD RED 6TB was reading and writing with a speed of aboud 113 MB/s

The Seagate IronWolf 8 TB was about 20% faster than the WD:

This is probably because the Seagate has 7200 rpm instead of 5400 rpm of the WD disk.

Disk temperature

One point in favour of the WD is the temperature. It is ~7-8 degrees colder than Seagate: 42° (WD) vs. 49° (Seagate).

Wipe data on old disks

Bevor we sell or trash the old disk I reccomend to wipe the data. I used the Delock Docking- and Clone-Station: