The mailbox disk is a small dedicated area on two disks in a cluster that is used for communication between the nodes. Each node can raise a flag on the other node's mailbox disk to indicate it is alive. This allows the nodes to monitor each other like email, hence the name "mailbox disk". The mailbox disk is typically located on the first aggregate and can be identified in the cluster monitor output. The mailbox disk may not upgrade automatically during disk firmware upgrades, so it needs to be upgraded separately on each node.

1. NetApp Mailbox disk
So called "mailbox disk" is used for clustering interconnection and communication between two
cluster "heads".
"Mailbox disk" it's a special area at a two of disk (by default one data and one parity, or one parity
and one dparity disks), and usually resides at a first aggregate.
It's consume a very small amount of space (around KB or two)
Actuallly, it's just a flag area, one "head" can raise the flag - "I'm alive! Everything is OK!" for another
one, partner "head", and vice-versa. It looks like e-mailing between heads, it's a reason why they
calling it "mailbox".
To identify the mailbox disk:-
Login to => Priv set advanced => cf monitor all
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netapp1> cf monitor all
cf: Current monitor status (19Jun2013 14:40:29):
partner 'netapp2', VIA Interconnect is up (link 0 up, link 1 up)
state UP, time 49476116655, event CHECK_FSM, elem ChkMbValid (12)
mirrorConsistencyRequired TRUE
takeoverByPartner 0x12000 <TAKEOVER_ON_PANIC>
mirrorEnabled TRUE, lowMemory FALSE, memio UNINIT, killPackets TRUE
degraded FALSE, reservePolicy ALWAYS_AFTER_TAKEOVER, resetDisks TRUE
timeouts:
fast 1000, slow 2500, mailbox 10000, connect 5000
operator 600000, firmware 10000 (recvd 49476116655), dumpcore 60000
booting 300000 (recvd 0)
transit timer enabled TRUE, transit 600000 (last 73383)
mailbox disks:
Disk 0a.17 is a local mailbox disk
Disk 0c.21 is a local mailbox disk
Disk 0a.22 is a local mailbox disk
Disk 0c.22 is a local mailbox disk
Disk 0b.17 is a partner mailbox disk
Disk 0d.22 is a partner mailbox disk
Disk 0b.16 is a partner mailbox disk
Disk 0d.16 is a partner mailbox disk
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Know issue with mailbox disk:-

2. When we perform disk firmware upgrade for all disk, these disk in most cases would not be
upgraded.
Solution:
Perform single disk upgrade for mailbox disk.
Copy downloaded fw to etc/disk_fw on both,(clustered),monitor console for disk upgrade proces.
any Issues::
raid.background_disk_fw_update.enable off.
Fallback for disk,
disable raid.background_disk_fw_update.enable off
>"disk_fw_update "hba-id.shelf-id.disk-number"
eg:
> disk_fw_update 1d.02.8