error occurred (0) - the controller has encountered read/write errors (the hdd has bad sectors) and should be replaced
offline member - for some reason the hdd is offline and not active in the raid so the hdd is unused, you can get with this one in raid 1 with mirror as you will still be able to boot.
If you get an offline member and don't do anything and then you get an error occurred (0) as i got on a Supermicro X7DB3 server you will not be able to boot in os anymore.
How to fix it ?
Remove the error occurred(0) hdd - you can try by removing one by one restart and check raid members hdd status, then access the raid utility by pressing CTRL+I
and you will be asked to reactive the offline member.
Next buy a new hdd to change the error occurred(0) hdd and it will rebuild the raid back, you can still boot to windows just with one hdd until you get a new one.