replace with different hard drive

Yes you can, although if you want the extra space to be added to the volume you'll need at least one additional disk with the higher capacity.
I'm assuming you are using xraid?

Hi

Yes I use XRaid, and I currentky have 4 3TB Hitachi drives, one has failed and I thought I replace it with a cheaper WD RED 3 TB disk instead.

Yes.
Do consider that you may need to adjust the idle timer using this tool from wd:
http://support.wd.com/product/download. ... 01&lang=en

Also consider that the seagate NAS drives are good, don't require this adjustment, and are slightly cheaper ( at least where I live)

Hmmm, is this WD upgrade because of mixed drives? The reason I ask is because I have another Ready NAS Unit where I only have WD Red 3TB drives (4 of them), and I haven't noticed any issues with these drives.... If I should do a firmware upgrade of the WD RED drives from the Ready NAS unit I will need some guidance on how this is done, do you know how or do you know any tutorials on this?

The older red drives don't seem to be affected, newer ones are.
The tool allows you to adjust the idle timer so that the drive does not accumulate load/ unload cycles which may eventually shorten drive life. This is controversial though.
The tool is installed on your PC and the drive needs to be attached to the PC to make the adjustment. I assume there are instructions on the link I gave you.

It's not a firmware upgrade and you can't do it on drives in the NAS.
If you want to look at the other drives, look at the load cycle count in the smart attributes. These you can see in the health section.

Ok, I thought it had to be done on the ReadyNAS Unit, when buying a new drive its now problem for me to connect it to my pc and do the firmware upgrade of the drive there, but if where to do it on the existing drives that right now are in use, then I guess I would be in trouble, would that require a factory reset in that way?

That's why I said you should check the load cycle count of the existing drives. Look at the smart information and you will see a number for load cycle count. Depending on whether you have disk spindown set and how often you power down the NAS it will have a count in the 100's to 1000's.

If it s ok on the existing drives then you don't need to adjust anything. I expect your existing drives will be ok.

Even if you needed to adjust, you wouldn't need to factory reset. Western digital does advise making a backup though before running the tool. You should have a backup anyway.

Ok, thanks. My first concern in this post was that one drive needed to be replaced, at least I thought so.. I got that message after I did a factory reset. But when I look now it says all my drives are ok!?

And also, I don't have the frontview gui in english, could you tell me what row number the "load cycle" is on?

I'd test the disk properly using vendor tools before assuming it's ok.

Thats a good idea, just ran the test and they where all ok, ...