The current pool should have all the data, it's raid1, and you should be able to just add the 2TB to the pool in place of the missing device.

but when i start the array either with or without the drive im getting 'unmountable wrong or no file system'

The pool was mounting in the last diags oposted:

/dev/sdc1 702G 408G 65G 87% /mnt/cache

If now it's not, need new diags.

server retarted and pool mounted with the two disks as before

server-diagnostics-20250905-1925.zip

I mentioned before that the pool won't mount with the other 2TB device unassigned, since it's part of the pool, drop offline again using the command above (device is currently /dev/sdf), then restart the array and see if the pool contents with just those two devices looks correct.

thankyou, that works,

how can i add the drive back into the pool then - or is that a lost drive...

If the pool contents look fine without that drive, reboot to bring it back, then before array start wipe it with blkdiscard -f /dev/sdX, then add it in place of the current pool missing device.

followed the above and, re-added the drive back to the array and still get unmountable.

server-diagnostics-20250907-1738.zip

Looks like you didn't wipe the other device. Try again, just mounting the pool with the other two devices, disconnect the 3rd one.

Ok so i apologize for being inpatient...

i did the above formatted the drive (btrfs) added it to the pool and re-started the server.

No access to GUI.

Ah i thought i know how to fix this... deleted config.cfg from pools. restarted the server.

Access back to gui Yeah!,

re-created the pool and added the three drives, restarted the server...

Unmountable :(

server-diagnostics-20250908-2002.zip

I don't say to format the drive, said to wipe it, if the pool still mounted with the other two.

Post again the output from btrfs fi show

opps didnt realsie the difference.

Label: none uuid: 01cc169c-ab3e-4f2b-86e3-5382b85f07e7

Total devices 3 FS bytes used 471.12GiB

devid 2 size 931.51GiB used 474.36GiB path /dev/sdc1

devid 4 size 238.47GiB used 238.47GiB path /dev/sdh1

*** Some devices missing

Label: none uuid: 8f7ab551-6fc8-455a-b371-6121ae81d696

Total devices 1 FS bytes used 144.00KiB

devid 1 size 1.82TiB used 2.02GiB path /dev/sdf1

52 minutes ago, Belcraig said:

opps didnt realsie the difference.

There is, but that wasn't the issue this time; you are not assigning the two current pool 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 2 slots

assign sdc and sdh only, leave the filesystem set to auto

start the array to import the pool and post new diags.

server-diagnostics-20250909-1715.zip

I've done as instructed..

Now stop the array, and add one of the unassigned cache devices in place of the missing device.

Start the array, wait for the balance to finish, and post new diags.

I stopped the array, it now shows three slots? when i select one of the disks in place of the missing device i get cache too many wrong or missing devices.

Pool has dual data profiles; convert to raid1, when it finishes, stop array, start the array, post new diags, still with only those two devices assigned, but you may need to reimport it again once that's corrected.

how to do this please...

https://forums.unraid.net/topic/46802-faq-for-unraid-v6/#findComment-480421

root@Server:~# btrfs balance start -dconvert=raid1 -mconvert=raid1 /mnt/cache

ERROR: error during balancing '/mnt/cache': Read-only file system

root@Server:~# dmesg | tail

[42390.025192] cdc_acm 3-12:1.0: ttyACM0: USB ACM device

[42393.486851] usb 3-12: USB disconnect, device number 35

[42393.790122] usb 3-12: new full-speed USB device number 36 using xhci_hcd

[42393.919642] cdc_acm 3-12:1.0: ttyACM0: USB ACM device

[45993.120095] usb 3-12: USB disconnect, device number 36

[45993.370983] usb 3-12: new full-speed USB device number 37 using xhci_hcd

[45993.500009] cdc_acm 3-12:1.0: ttyACM0: USB ACM device

[45996.961219] usb 3-12: USB disconnect, device number 37

[45997.271974] usb 3-12: new full-speed USB device number 38 using xhci_hcd

[45997.401033] cdc_acm 3-12:1.0: ttyACM0: USB ACM device

If the the filesystem is going read-only during balance, there are other problems. I suggest you backup what you can from the pool and reformat it.