For a Western Digital HD to have that many Seek Errors, it usually means that this drive is not in good shape anymore... The drive is still clear on metrics No 5, 197 and 198... So the drive is having a hard time to properly positioned its heads at the right place on the disk (seek errors). Because of that, it must keep trying again and again before getting it right.
I do not have WD drives myself, so do not know much more about how they behave. What I do know is that this metric is not measured the same way in Seagate drives like the IronWolf I use here.
have turned the NAS off for now until i can get a replacement drive, just in case,
Know that this is not always the best idea. To let the drive cool down and park for a long time may lead them to never restart again... Redundancy, cold spares and backups is what will protect you better against such a case.
Here, I ended up with a failed smart test before Christmas. I replaced the drive with my cold spare, so the pool is back healthy. Any drive can fail and redundancy will keep my server up. Should a second drive fail, I will still be OK as long as it is in a different mirror. Should a complete mirror fail, then I will have to re-transfer my data from the snapshots I sent to my 2 other servers. Should I end up with a degraded pool and no more spares on site, I would keep Thanatos Online all the time to ensure the latest snapshots are available onsite. Once the pool is back to healthy and spares are in, Thanatos would go back to sleep.
In all cases, do your backups now if you do not already have any. Never too soon to make some backup, but often people try to do them too late.