The error is seen when examining the disk.

mdadm --examine /dev/sdb

returns:

mdadm: No md superblock detected on /dev/sdb

Check the other disks within the RAID Array:

mdadm --examine /dev/sde

returns:

/dev/sde:
          Magic : a92b4efc
        Version : 00.90.00
           UUID : f781ba39:3975fa64:c36d41f1:ea056919
  Creation Time : Mon Jan  5 14:18:27 2009
     Raid Level : raid5
  Used Dev Size : 976762496 (931.51 GiB 1000.20 GB)
     Array Size : 2930287488 (2794.54 GiB 3000.61 GB)
   Raid Devices : 4
  Total Devices : 4
Preferred Minor : 0
 
    Update Time : Thu Mar 19 22:39:52 2009
          State : clean
 Active Devices : 4
Working Devices : 4
 Failed Devices : 0
  Spare Devices : 0
       Checksum : 54f44664 - correct
         Events : 0.92
 
         Layout : left-symmetric
     Chunk Size : 64K
 
      Number   Major   Minor   RaidDevice State
this     3       8       32        3      active sync   /dev/sdc
 
   0     0       8       16        0      active sync   /dev/sdb
   1     1       8       64        1      active sync   /dev/sde
   2     2       8       48        2      active sync   /dev/sdd
   3     3       8       32        3      active sync   /dev/sdc

mdadm --stop /dev/md0

returns:

mdadm: stopped /dev/md0

mdadm --assemble --scan -v

returns:

mdadm: /dev/sde1 is identified as a member of /dev/md0, slot 2.
mdadm: /dev/sdf1 is identified as a member of /dev/md0, slot 3.
mdadm: /dev/sdd1 is identified as a member of /dev/md0, slot 0.
mdadm: /dev/sdb1 is identified as a member of /dev/md0, slot 1.
mdadm: added /dev/sdd1 to /dev/md0 as 0 (possibly out of date)
mdadm: added /dev/sdb1 to /dev/md0 as 1 (possibly out of date)
mdadm: added /dev/sdf1 to /dev/md0 as 3 (possibly out of date)
mdadm: added /dev/sde1 to /dev/md0 as 2
mdadm: /dev/md0 assembled from 1 drive - not enough to start the array.

If the previous command fails, try to repeat the assemble with the –run or –force options:

mdadm --assemble --scan -v --force

or

mdadm --assemble --scan -v --run

If the Array is still not working, try to recreate the array:

mdadm create /dev/md0 --assume-clean <original create options> /dev/sd[dbfe]1.

https://ahelpme.com/linux/inactive-array-mdadm-cannot-get-array-info-for-dev-md126/

https://www.linuxquestions.org/questions/linux-general-1/recovering-mdadm-superblocks-713234/