Starting with 2025 non certified disks cannot be used on a Synology running DSM. If you try to repair a storage pool the non certified disks will not show up in the storage manager and therefore cannot be selected for a storage pool. In the German magazin c't 2025/14 Mirko Dölle wrote an article which explains in detail how to repair a storage pool with non certified disks and SSDs on a Synology on the command line. The following article just summarizes the required commands without any detailed explanations to remind me how to get this done when I need these commands in the future.

Check which RAID failed:

cat /proc/mdstat

mdadm -v --detail --scan

If a disk cannot be used at all the device in the mdadm list will be missing. Remove the disk now with assuming /dev/md2 is the  RAID to repair:

mdadm --manage /dev/md2 --remove /dev/nvme1n1p3

 Now either replace the disk is hotswap is awailt. Otherwise shut down the Sysnology and replace the disk.

Check for device names:

blkid | grep "/dev/[sn]"

Now check which VG uses the defect disk:

pvdisplay -m

Let's assume it's VG2 in following command to partition the new disk:

synospace --partition --disk-init --add-to=/dev/vg2 /dev/nvme1n1

Check now there exists at least three partitions:

blkid | grep "/dev/[sn] | sort"

Now add all missing partitions to the RAID. If there is no DSM on the disk it's just the last partition. Otherwise add also the first two paritions accordingly:

mdadm --manage /dev/md2 --add /dev/nvme1n1p5

Now the RAID will start the RAID rebuild.

Note: These steps work as now on 6/29/2025 but may no longer work in the future due to DSM modifications by Synology.