raidz3 degraded but how ?

Hello,

I have a degraded raidz3 pool but can't figure out how to fix it. I have already replaced 4 disks successfully but the zpool status is confusing and I simply don't know which disk to replace... In fact I created the pool originally with 24 HDs and the status reports 24 ONLINE drives, see below:

$ zpool status
  pool: tank
 state: DEGRADED
status: One or more devices has been taken offline by the administrator.
        Sufficient replicas exist for the pool to continue functioning in a
        degraded state.
action: Online the device using 'zpool online' or replace the device with
        'zpool replace'.
  scan: resilvered 871G in 42h11m with 0 errors on Sun Dec 16 10:59:24 2018
config:

        NAME STATE READ WRITE CKSUM
        tank DEGRADED 0 0 0
          raidz3-0 DEGRADED 0 0 0
            sdb ONLINE 0 0 0
            sdc ONLINE 0 0 0
            sdd ONLINE 0 0 0
            sde ONLINE 0 0 0
            sdf ONLINE 0 0 0
            sdg ONLINE 0 0 0
            sdh ONLINE 0 0 0
            sdi ONLINE 0 0 0
            sdj ONLINE 0 0 0
            sdk ONLINE 0 0 0
            sdl ONLINE 0 0 0
            sdm ONLINE 0 0 0
            sdn ONLINE 0 0 0
            sdo ONLINE 0 0 0
            sdp ONLINE 0 0 0
            sdq ONLINE 0 0 0
            sdr ONLINE 0 0 0
            sdw ONLINE 0 0 0
            sdt ONLINE 0 0 0
            sdu ONLINE 0 0 0
            sdv ONLINE 0 0 0
            spare-21 DEGRADED 0 0 0
              sdw OFFLINE 0 0 0
              sds ONLINE 0 0 0
            sdab ONLINE 0 0 0
            sdad ONLINE 0 0 0
        spares
          sds INUSE currently in use
          sdab FAULTED corrupted data

$ lsb_release -a
LSB Version: core-9.20160110ubuntu0.2-amd64:core-9.20160110ubuntu0.2-noarch:security-9.20160110ubuntu0.2-amd64:security-9.20160110ubuntu0.2-noarch
Distributor ID: Ubuntu
Description: Ubuntu 16.04.5 LTS
Release: 16.04
Codename: xenial

$ uname -a
Linux tullus 4.11.0-14-lowlatency #20~16.04.1-Ubuntu SMP PREEMPT Wed Aug 9 10:04:55 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

$ dpkg -l | grep zfs
ii libzfs2linux 0.6.5.6-0ubuntu26 amd64 Native OpenZFS filesystem library for Linux
ii zfs-doc 0.6.5.6-0ubuntu26 all Native OpenZFS filesystem documentation and examples.
ii zfs-zed 0.6.5.6-0ubuntu26 amd64 OpenZFS Event Daemon (zed)
ii zfsutils-linux 0.6.5.6-0ubuntu26 amd64 Native OpenZFS management utilities for Linux