So I have a situation where the array is 1 3TB data 1 3TB parity.

The data drive is failing and freezing parity check around 75%.

I cannot readily find 3TB disks. So I am getting 2 x 4TB disks.

BUT I need to replace my data disk first - because I trust the parity disk - THEN replace the parity.

AFAIK I cannot add a larger than parity data disk - nor can I use parity swap, as I don't want new parity to read from my data disk.

What to do?

11 minutes ago, NLS said:

nor can I use parity swap, as I don't want new parity to read from my data disk.

Did you actually click the parity swap link and read the documentation about that? It doesn't read your data disk. It is exactly for this situation. The data disk can be completely destroyed and you can still do parity swap.

What @trurl said is true, but in your case there is an extra option as with an array consisting of exactly one data drive and one parity it is a special case where the parity drive is a mirror of the data drive.

You could instead do Tools->New Config, and assign the new 4TB drive to parity and the old 3TB parity drive as disk1. When you start the array it will simply build new parity and the 3TB drive will be intact with all your data.

If you intend to use the other 4TB drive as an additional data drive (rather than replacing a 3TB drive) then you could add it at the same time after doing New Config and the parity that is built will cover both data drives.

Thanks to both.

On 4/14/2026 at 6:59 PM, trurl said:

Did you actually click the parity swap link and read the documentation about that? It doesn't read your data disk. It is exactly for this situation. The data disk can be completely destroyed and you can still do parity swap.

I am a bit confused...
Remember this specific setup has just two disks (parity+data).
I did the procedure described in parity swap.
Indeed it started to copy from old parity to new parity, which finished early in the morning.
So now it shows array as stopped.
Here is where the weird thing starts:
- If I unassign from data slot the old parity (so the system is "just parity" for a while to see if it comes online), it says "too many disks replaced/missing". Which is weird as I expected the system to come online with an emulated data disk from (new) parity.
- If I assign the old parity (that is already copied) back to data slot, it does NOT treat it as "new" data disk, I still get the "copy" button! It cannot reconstruct an "new" data disk on top of the old parity (after all even with 4TB parity the actual data are from a 3TB parity).
This is a bit strange, I expected UNRAID to be able to treat the old parity as new "to use and rebuild from (new) parity" data disk. Apparently not?

...so now I have to replace the old parity with a new empty data disk and HOPE this will rebuild the new data from copied parity finaly. AT LEAST I HOPE this is what it needs now. I do have that extra disk (4TB) luckily.
This is not explained in the parity swap guide and as the instructions are now, it reems like I could bring array online after the copy...

So until I visit the place and do the replacement of data disk (in a couple of hours), I hope someone can make me relax by saying this is expected situation.
It is the first parity swap in a 2 disk system, in more than a decade I handle UNRAID setups.

FOLLOW UP: NO GO.

I unmarked the solution.

Does not work properly with two disks.

After the above. I removed the old parity, installed new to-be-data disk.

Initially I tried with only parity drive awsigned. The new copied parity. It didn't allow me to start.

Then I tried to assign new data disk. No go. Too many changed.

Then I selected new config. This allowed minimum of 3 disks. 2 parity and 1 data. I assigned new (copied) parity to parity and the empty data disk to data. I clicked that parity is valid. It still said it would overwrite parity and destroy it's data. So, no go.

Replaced both old disks back and for now started the system as it was before parity swap.

This method is no go with 2 disks and always 2 disks. (might work differently if I added a 3rd disk... Maybe 2nd parity)

So I am still looking for a method that works.

Parity swap definitely works with just two devices, note that once the parity copy finishes, you cannot do anything else (or reboot) other than just start the rebuild.

If that is failing, post the diags after the attempt.

4 minutes ago, JorgeB said:

Parity swap definitely works with just two devices, note that once the parity copy finishes, you cannot do anything else (or reboot) other than just start the rebuild.

If that is failing, post the diags after the attempt.

I cannot post diags since I am several steps (and reboots) after that.

I can tell you that copy finished seemingly successfully (see photo),

...this left the array in a stopped state. No errors.
Also no way to start the array after that:

• Neither "as is" (with new parity copy and old parity disk being in data disk position assigned), to render the old parity as new data. There was no start button. Only a "copy" button still. I think it was clickable (like copy never happened - in contrast with having that status message above and me actually SEEING the copy happening until almost 20% -then had to leave it continue unattended).

  \

  

  (above 2 screenshots are AFTER copy seemingly finished)

• Or by unassigning old parity and stay with copied parity and no data disk so that data are "emulated".

...so I was forced to reboot, as there was nothing to do with the above setup. And the rest is as I describe in my previous message.
I tried to replace the old parity disk (that was in a data possition as "parity swap" dictates) with new fresh data disk. No go.

As you can see (the below is just before I eventually tried "new config") it ignored the copy it (seemingly) performed over several hours that finished WITHOUT error and shows parity as "wrong". Somehow it expected the old parity in place!!!

...after this I was forced to make "new config", that as I said, demanded I rebuild my (copied - valid?) parity using the data from the... empty new data disk. Of course I didn't allow that and reverted to both OLD disks (old parity, old data, accept parity as valid). So now they are back to square one.

Edited April 21Apr 21 by NLS

42 minutes ago, NLS said:

Neither "as is" (with new parity copy and old parity disk being in data disk position assigned), to render the old parity as new data. There was no start button. Only a "copy" button still.

After the copy is finished successfully, it will show the "start data rebuild" option, and you cannot click on anything else.

If it's not showing that, it suggests the copy is failing. You can try again, and if after the copy finishes you don't see the rebuild option, post the diags.

9 minutes ago, JorgeB said:

After the copy is finished successfully, it will show the "start data rebuild" option, and you cannot click on anything else.

If it's not showing that, it suggests the copy is failing. You can try again, and if after the copy finishes you don't see the rebuild option, post the diags.

Yeah, never showed that. This indicates a bug, as if it failed it should say so. The only message I got, I showed you above.

Thing is what you ask to do, takes several hours (around 8-9 I think it took) and without any change, I suspect I will end up with the same (non) result.

Question is, would itimpi proposal work maybe?

In two disk setup the parity is a mirror or a "negative" mirror? Because if it is a mirror (and NOT a negative mirror), I can simply re-assign it as data and add a fresh larger parity instead (with new config).
Anybody can verify?

---

EDIT: I suspect it would not work and parity would just be XOR contents.
Also adding a (temporary) second parity (the new larger disk) probably will not help as it will try to use old parity and old (unreliable and freezing) data disk to rebuild second parity. So, this doesn't help. I am at a loss.

Edited April 21Apr 21 by NLS

33 minutes ago, NLS said:

Question is, would itimpi proposal work maybe?

Yes, that should also work.

6 hours ago, JorgeB said:

Yes, that should also work.

So, parity in a dual disk setup is really a mirror?

Yep, you can just assign old parity as disk1 after a new config.