Hello community,

I recently purchased a 4TB SSD to be used for my cache pool. I already had a cache pool set up (btrfs), consisting of 2 1TB SSDs using the balancing mode so both drives appear as one device so to speak. I'm on Unraid 7.2.

My question: how do I replace the 2 1TB SSDs (which have about 1.4TB of data on them; appdata, system, lancache and some eBooks) with the 4TB SSD, data transfer and all? I just want the 4TB to be used for the pool now, as I'm going to retire the 2 1TB SSDs to my personal system.

So far, I've added the 4TB SSD to the current pool and rebalanced the content.
Here is a screenshot of the balance status:


If I read the btrfs info correctly, all data in the pool is currently on disk 3, aka the 4TB one?
I tried stopping the array, unassigning the other 2 disks and starting with just disk 3 in the cache pool, but Unraid wouldn't let me, since the cache pool was missing disks (even with ticking "Yes, I want this").

If at all possible I would want to avoid moving all contents to my array, removing the pool, creating a new one, and then moving the stuff back, as we are talking about 1.4TB x 2 amount of read and write, which not only takes a long time but it also introduces a lot of unnecessary writing operations.

Thank you in advance.

Edited December 10, 2025Dec 10 by Fortesque

You can convert the pool to raid1, then remove one of the devices at a time.

Hi JorgeB,
Thank you for your reply. Since it's a lot of data, I just want to be safe about this. Therefore, forgive me if my questions/assumptions are somewhat stupid: wouldn't raid1 mirror all data to all disks? How would that work if the data total is 1.4TB but both of the older SSDs are just 1TB in size? Also I'm not sure about the "remove one of the devices at a time" part. After successful conversion to raid1, do I physically remove one of the SSDs or do I remove one from the pool by unassigning the disk?

Edited December 11, 2025Dec 11 by Fortesque

1 hour ago, Fortesque said:

wouldn't raid1 mirror all data to all disks?

With btrfs it just means there must be two copies of all blocks; they will be distributed by the available devices, you would have 2TB usable in raid1 mode.

See below for how to remove devices from a redundant pool (you cannot remove them from a single profile pool)

https://forums.unraid.net/topic/46802-faq-for-unraid-v6/#findComment-480418

Just now, JorgeB said:

you would have 2TB usable in raid1 mode.

Wait a second, I failed to consider that when you remove the first device, there won't be enough space, so you cannot use that approach unless you remove some data from the pool to make it less than 1TB used.

That's what I thought! Is there a way (raid configuration, ...) to use the 4TB device as a redundancy/parity disk, then remove the 1TB devices and then just use the redundant data as the actual data? Seems to me that migrating data (even inside a pool) from 2 smaller devices to a bigger one should be a common case.

2 hours ago, Fortesque said:

to use the 4TB device as a redundancy/parity disk, then remove the 1TB devices and then just use the redundant data as the actual data?

Not using the GUI, you can still do it manually. Manually, you can remove devices from BTRFS non-redundant pools, then need to reimport the pool.

2 hours ago, Fortesque said:

Seems to me that migrating data (even inside a pool) from 2 smaller devices to a bigger one should be a common case.

Typically people upgrade mirrors, and that would not be a problem.