PR4100 with group descriptors corrupted!

My Cloud OS 3 Personal & Network Attached Storage My Cloud Pro Series
45

This is what I get from mdadm, which sounds about right, 24Tb of which 15TB was used:

sudo mdadm -D /dev/md1

/dev/md1:
Version : 01.00
Creation Time : Tue Jan 9 20:42:25 2018
Raid Level : raid5
Array Size : 23429492160 (22344.11 GiB 23991.80 GB)
Used Dev Size : 15619661440 (14896.07 GiB 15994.53 GB)
Raid Devices : 4
Total Devices : 4
Preferred Minor : 1
Persistence : Superblock is persistent

Intent Bitmap : Internal

Update Time : Sat Mar 28 06:06:54 2020
      State : clean

Active Devices : 4
Working Devices : 4
Failed Devices : 0
Spare Devices : 0

     Layout : left-symmetric
 Chunk Size : 64K

       Name : 1
       UUID : 7b532c3f:a9089a56:57b177b6:a8f67641
     Events : 153

Number   Major   Minor   RaidDevice State
   0       8        2        0      active sync   /dev/sda2
   1       8       18        1      active sync   /dev/sdb2
   2       8       34        2      active sync   /dev/sdc2
   3       8       50        3      active sync   /dev/sdd2
46

Tried to mount it:

# mount /dev/md1 /shares/Volume_1/ -t ext4
mount: wrong fs type, bad option, bad superblock on /dev/md1,
       missing codepage or helper program, or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

looks like its still corrupt:

[15381181.424069] EXT4-fs (md1): ext4_check_descriptors: Block bitmap for group 21568 not in group (block 2251799813718436)!

    [15381181.436332] EXT4-fs (md1): group descriptors corrupted!

    [15381235.715331] EXT4-fs (md1): ext4_check_descriptors: Block bitmap for group 21568 not in group (block 2251799813718436)!

    [15381235.727608] EXT4-fs (md1): group descriptors corrupted!
48

I’m sorry to read this… but judging from the log file you share earlier, it doesn’t look good.
I think it’s beyond saving…
My next step would be to use some forensic recovery software… or this last resort command: ext4 - Recover from a corrupted filesystem when fsck do not help - Ask Ubuntu

My recommendation is to get professional help though.