Synology, trying to bring back a failed RAID array

Question

so I manage to destroy some of my data... I have a ds916+ and would like to recover the data from the drives, I ordered a new drive because one was failing, and another one failed during resync... so ye, double disk failure.

Here I'm now:

root@NAS:~# mdadm --detail /dev/md2
/dev/md2:
        Version : 1.2
  Creation Time : Sat Mar  3 14:15:17 2018
     Raid Level : raid10
     Array Size : 3897385088 (3716.84 GiB 3990.92 GB)
  Used Dev Size : 1948692544 (1858.42 GiB 1995.46 GB)
   Raid Devices : 4
  Total Devices : 3
    Persistence : Superblock is persistent

    Update Time : Wed Apr 14 21:28:42 2021
          State : clean, FAILED
 Active Devices : 3
Working Devices : 3
 Failed Devices : 0
  Spare Devices : 0

         Layout : near=2
     Chunk Size : 64K

           Name : NAS:2  (local to host NAS)
           UUID : 928d46c6:72c08f44:8997f935:c9c219cf
         Events : 11818

    Number   Major   Minor   RaidDevice State
       0       8        3        0      active sync set-A   /dev/sda3
       1       8       19        1      active sync set-B   /dev/sdb3
       -       0        0        2      removed
       3       8       51        3      faulty active sync set-B   /dev/sdd3

Now, when I do a:

mdadm --manage /dev/md2 --add /dev/sdc3

This drive is being added as a spare:

root@NAS:~# mdadm --detail /dev/md2
/dev/md2:
        Version : 1.2
  Creation Time : Sat Mar  3 14:15:17 2018
     Raid Level : raid10
     Array Size : 3897385088 (3716.84 GiB 3990.92 GB)
  Used Dev Size : 1948692544 (1858.42 GiB 1995.46 GB)
   Raid Devices : 4
  Total Devices : 4
    Persistence : Superblock is persistent

    Update Time : Wed Apr 14 21:28:12 2021
          State : clean, FAILED
 Active Devices : 3
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 1

         Layout : near=2
     Chunk Size : 64K

           Name : NAS:2  (local to host NAS)
           UUID : 928d46c6:72c08f44:8997f935:c9c219cf
         Events : 11817

    Number   Major   Minor   RaidDevice State
       0       8        3        0      active sync set-A   /dev/sda3
       1       8       19        1      active sync set-B   /dev/sdb3
       -       0        0        2      removed
       3       8       51        3      faulty active sync set-B   /dev/sdd3

       4       8       35        -      spare   /dev/sdc3

What I would like to achieve, is that the sdc3 will jump back to it's place and with that I hope I get access to some of the data, the drive was working before replacement.

Also, the following mdadm command doesn't work:

root@NAS:~# mdadm --manage /dev/md2 --re-add /dev/sdc3
mdadm: --re-add for /dev/sdc3 to /dev/md2 is not possible

Here is also some additional information which may help:

root@NAS:~# mdadm --examine /dev/sdc2
/dev/sdc2:
          Magic : a92b4efc
        Version : 0.90.00
           UUID : fea95269:e7e6ab4e:cced5de7:ca715931 (local to host NAS)
  Creation Time : Sat Jan  1 01:00:09 2000
     Raid Level : raid1
  Used Dev Size : 2097088 (2047.94 MiB 2147.42 MB)
     Array Size : 2097088 (2047.94 MiB 2147.42 MB)
   Raid Devices : 4
  Total Devices : 4
Preferred Minor : 1

    Update Time : Wed Apr 14 21:09:33 2021
          State : clean
 Active Devices : 4
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 0
       Checksum : c01e878e - correct
         Events : 158


      Number   Major   Minor   RaidDevice State
this     2       8       34        2      active sync   /dev/sdc2

   0     0       8        2        0      active sync   /dev/sda2
   1     1       8       18        1      active sync   /dev/sdb2
   2     2       8       34        2      active sync   /dev/sdc2
   3     3       8       50        3      active sync   /dev/sdd2

root@NAS:~# fdisk -l /dev/sd[a-d]
Disk /dev/sda: 1.8 TiB, 2000398934016 bytes, 3907029168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0x27fe7bf8

Device     Boot   Start        End    Sectors  Size Id Type
/dev/sda1          2048    4982527    4980480  2.4G fd Linux raid autodetect
/dev/sda2       4982528    9176831    4194304    2G fd Linux raid autodetect
/dev/sda3       9437184 3906824351 3897387168  1.8T fd Linux raid autodetect
Disk /dev/sdb: 1.8 TiB, 2000398934016 bytes, 3907029168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0x26a4ac9c

Device     Boot   Start        End    Sectors  Size Id Type
/dev/sdb1          2048    4982527    4980480  2.4G fd Linux raid autodetect
/dev/sdb2       4982528    9176831    4194304    2G fd Linux raid autodetect
/dev/sdb3       9437184 3906824351 3897387168  1.8T fd Linux raid autodetect
Disk /dev/sdc: 1.8 TiB, 2000398934016 bytes, 3907029168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0x7eba1232

Device     Boot   Start        End    Sectors  Size Id Type
/dev/sdc1          2048    4982527    4980480  2.4G fd Linux raid autodetect
/dev/sdc2       4982528    9176831    4194304    2G fd Linux raid autodetect
/dev/sdc3       9437184 3906824351 3897387168  1.8T fd Linux raid autodetect
Disk /dev/sdd: 1.8 TiB, 2000398934016 bytes, 3907029168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0xea4cb292

Device     Boot   Start        End    Sectors  Size Id Type
/dev/sdd1          2048    4982527    4980480  2.4G fd Linux raid autodetect
/dev/sdd2       4982528    9176831    4194304    2G fd Linux raid autodetect
/dev/sdd3       9437184 3906824351 3897387168  1.8T fd Linux raid autodetect

Please help! :)