Hi all, and a happy new year.

I have 2 faulty SSD's in my 3 disk cache pool that I need to replace. To be on the safe side I added a new drive to the pool before trying to remove a faulty one. After letting BTRFS balance the data and running a scrub I tried to remove one of the the faulty drives by un-assigning it from the pool. This didn't work as expected.

This is the pool after I added the new device and BTRFS was done with re-balancing and scrubbing:

I then stop the array and un-assign the faulty drive (sdd);

However, when I start the array, the pool is broken:

When I tried to stop the array it hung indefinitely on "Unmounting disks" and I had to reboot the system into safe mode, delete the config for the cache from /boot/config/pools, and restart the system again.

I imported the 3 disks I want to keep in the pool but then I get this error when I start the array:

I've read a couple of threads on this topic but I'm not comfortable going forward without guidance as I haven't backed up my cache in a while and would prefer not to screw things up further. 😅

I'm not sure which version of the diagnostics that's most useful but I've included the one where the pool is in the state as shown in the image directly above. I've also included information from "btrfs fi show" as that was requested in a thread similar to this.

Any help is greatly appreciated.

btrfs fi show.txt nas-diagnostics-20260101-1133.zip

7.0.x has a bug removing pool devices; fixed in 7.1 and later.

To correct the issue, with the array running type:

btrfs device remove missing /mnt/cache

Once that finishes, reimport the pool with the remaining 3 devices:

on main click on the first device for that pool and then "remove pool"

back on main, create a new pool with the same name and 3 slots

assign the 3 pool devices, leave the filesystem set to auto

start the array to import the pool

Thanks Jorge, that did the trick. I debated if I should update to 7.2 but wasn't sure if that'd make things worse. I'll do that before I change the next pool disk though.

For anyone else using this thread as guidance, the "btrfs device remove missing /mnt/cache" command took quite a while to complete.

Edited January 1Jan 1 by MartinNr5
Clarified last sentence.