Noob needs to know about swapping larger disks into FreeNAS

Hi. Complete FreeNAS noob here. I built 2 machines using old motherboards and PC cases. Neither box holds more than 2 physical drives.

The first used two 500GB drives just so I could learn and get FreeNas working. The intention is that it will be my always on file server and backup machine for my PC. It’s been up and working and is great, but the disks are small.

The second machine has 4TB drives and it’s ONLY purpose is to back up the first machine. I turn it on once or twice a week, do a replication task, and it’s working great.

I bought 2 more 4TB drives which I want to put in the first machine. Can someone point me toward the accepted best way to do this? Do I fail one of the existing drives, remove it physically, put in a 4TB drive and then it rebuilds itself, and then do that again with the other 500GB drive, and when it’s done I have a 4TB pool, or do I have to do something more complicated?

Any pointers to how to do this are much appreciated.


I have never done any testing but I have heard there is a way to do it by replacing all the drives in the pools. If no one here has an answer try their forums TrueNAS Community

Thanks for the reply. Most the advise there is to follow the Guide which says you can fail a drive and replace it with the same size or larger. What it doesn’t seem to say is once all the drives are replaced, in my case just 2 drives, will the pool use all the new disk space.

Being that machine #2 is fully backing up (I think, I think, I really should test somehow) machine #1 I don’t think I have much risk of losing anything but I haven’t found a definitive answer yet.

Again, thanks for the reply.

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I’ve done this a few times… Mark the drive as offline, pull it out, install the new drive, refresh the drives so it sees the new drive, replace the “failed” drive and wait for it to resilver.

Do the same for the next drive until you are done swapping. The vdev should automatically grow to the largest permitted when the last drive is done resilvering.

I don’t think you can go smaller, but you can go to the same size with this process too.

Thanks to both you and Tom for your replies.

I tried your suggested process this morning and ran into a problem on the ‘replace’ step. My disk setup is a mirror, not a RAIDZ, and the replace step does not give me the name of the drive to replace so it didn’t work. I put the old drive back in the box and did a scrub and everything appears to be back to normal for now.

I cannot find anything about replace not working for mirrors in the Guide but really there isn’t much info about replacing drives at all. Watching a couple of videos on YouTube it looks like with mirrors the way I would do this is to first ‘detach’ one drive and then ‘extend’ the remaining 1-drive mirror with a new larger drive. Once resilvered I would do that again and the size of the pool would be the full size.

If you or anyone else reads this soon and has inputs I’ll probably wait a day to see if I get confirmation and then go ahead.


I’m trying to remember the specifics, but I needed to do something to see the new disk. You may need to go to disks and look to see if the new one is listed, I also cleaned/wiped mine, then went back to the pool and was able to select it in replace. Something about refreshing.

But as you say, mine are all RAIDZx and your mirrors could be different. I don’t have any mirrors to test on and no computers that I can even build a test machine on or I probably would try this for my own knowledge.

Sorry but I wasn’t clear enough. I could see the disk in the list. However when you got to the replace button it asks you what you are replacing. In my case the list was empty. Originally I had ada0p2 and ada2p2. ada1 is a log disk in case you think it matters. I took ada2p2 offline, powered down and switched the disk. After powering back up I could see the new disk with some long GUID-ish name. However when you do the replace it’s supposed to show you a list of disks you can replace but in my case I just see “—”.

As I say, I found a couple of YouTube videos where people originally set up 1 disk mirrors with the intention of adding redundancy later. In their case they do an extend because there’s nothing to replace. My assumption here is that a ‘detach’ operation takes a disk that’s part of the mirror and removes it but I’ve not verified that yet.


Hmmm, that’s strange but might be a characteristic of mirrors.

Possibly. No matter how I tried I never got past the ‘—’ at the point where I was supposed to choose the drive to replace. I decided this morning to handle it like I had booted the machine and a drive wasn’t there. Did a Detach and Remove, rebooted with a new 4TB WD Red Plus, extended the mirror, attached the new drive. It was easy and the resilvering is in progress so I think I’m good to go as long as the new drive ends up being reliable.

Thanks for your help along the way. It helped me get my bearings straight.