How do I reactivate my MDADM RAID5 array?

Solution 1:

The S labels means the disk is regarded as "spare". You should try stopping and re-starting the array:

  mdadm --stop /dev/md0
  mdadm --assemble --scan

to re-assemble the array and if that doesn't work, you may need to update your mdadm.conf, see for example this question for details on how to do that.

Solution 2:

This question is a bit old, but the answer might help someone facing a similar situation. Looking at the event counts from the mdadm --examine output you have provided, they seem close enough (955190 - for sdb1 and sdf1, 955219 for sde1 and for sdd1 you've got 955205). If they are below 40-50, this is OK, and in that case the recommended course of action is to assemble your array manually, forcing mdadm to accept the drives despite the event count difference:

Stop the array:

mdadm --stop /dev/md0

Then try to reassemble the array manually:

mdadm --assemble --force /dev/md0 /dev/sdb1 /dev/sdd1 /dev/sde1 /dev/sdf1

Check the status of the array, to examine if the drive list/structure is OK (bottom of command output will show which drive is at what status and at which position in the array):