synology discover removed drive and readd

Tim_DS411j> cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
md3 : active raid5 sdc6[0] sdb6[2]
      3906989568 blocks super 1.2 level 5, 64k chunk, algorithm 2 [3/2] [U_U]

md2 : active raid5 sdb5[4] sdc5[2] sdd5[3] sda5[1]
      5846338944 blocks super 1.2 level 5, 64k chunk, algorithm 2 [4/4] [UUUU]

md1 : active raid1 sda2[0] sdb2[1] sdc2[2] sdd2[3]
      2097088 blocks [4/4] [UUUU]

md0 : active raid1 sda1[0] sdb1[3] sdc1[2] sdd1[1]
      2490176 blocks [4/4] [UUUU]

unused devices: <none>

Tim_DS411j> mdadm --detail /dev/md3
/dev/md3:
        Version : 1.2
  Creation Time : Sun Oct  6 12:36:54 2013
     Raid Level : raid5
     Array Size : 3906989568 (3726.00 GiB 4000.76 GB)
  Used Dev Size : 1953494784 (1863.00 GiB 2000.38 GB)
   Raid Devices : 3
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Mon Jan 27 22:31:08 2020
          State : clean, degraded
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

         Layout : left-symmetric
     Chunk Size : 64K

           Name : Tim_DS411j:3  (local to host Tim_DS411j)
           UUID : 34dcc45e:ce93e311:313f9801:b01f7f8b
         Events : 5928778

    Number   Major   Minor   RaidDevice State
       0       8       38        0      active sync   /dev/sdc6
       1       0        0        1      removed
       2       8       22        2      active sync   /dev/sdb6


Tim_DS411j> mdadm --examine /dev/sdd6
/dev/sdd6:
          Magic : a92b4efc
        Version : 1.2
    Feature Map : 0x0
     Array UUID : 34dcc45e:ce93e311:313f9801:b01f7f8b
           Name : Tim_DS411j:3  (local to host Tim_DS411j)
  Creation Time : Sun Oct  6 12:36:54 2013
     Raid Level : raid5
   Raid Devices : 3

 Avail Dev Size : 3906989856 (1863.00 GiB 2000.38 GB)
     Array Size : 7813979136 (3726.00 GiB 4000.76 GB)
  Used Dev Size : 3906989568 (1863.00 GiB 2000.38 GB)
    Data Offset : 2048 sectors
   Super Offset : 8 sectors
          State : clean
    Device UUID : d0464cff:1c43bcee:89dcfce6:18736c35

    Update Time : Thu Jan 23 22:33:00 2020
       Checksum : c98ea4f1 - correct
         Events : 5928610

         Layout : left-symmetric
     Chunk Size : 64K

   Device Role : Active device 1
   Array State : AAA ('A' == active, '.' == missing)


Tim_DS411j> mdadm --add /dev/md3 /dev/sdd6
mdadm: re-added /dev/sdd6


Tim_DS411j> cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
md3 : active raid5 sdd6[1] sdc6[0] sdb6[2]
      3906989568 blocks super 1.2 level 5, 64k chunk, algorithm 2 [3/2] [U_U]
      [>....................]  recovery =  0.0% (285484/1953494784) finish=3192.7min speed=10195K/sec

md2 : active raid5 sdb5[4] sdc5[2] sdd5[3] sda5[1]
      5846338944 blocks super 1.2 level 5, 64k chunk, algorithm 2 [4/4] [UUUU]

md1 : active raid1 sda2[0] sdb2[1] sdc2[2] sdd2[3]
      2097088 blocks [4/4] [UUUU]

md0 : active raid1 sda1[0] sdb1[3] sdc1[2] sdd1[1]
      2490176 blocks [4/4] [UUUU]

unused devices: <none>