What are all these partitions on my Mac? I know the purpose of "EFI", "Boot OS X", and "Recovery HD" partitions. But why are there three different "Macintosh HD" partitions? And should there be two EFI partitions?



UPDATE

Here is what disktutil list reports

/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *121.3 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:          Apple_CoreStorage                         121.0 GB   disk0s2
   3:                 Apple_Boot Boot OS X               134.2 MB   disk0s3
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *3.0 TB     disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2:          Apple_CoreStorage                         2.7 TB     disk1s2
   3:                 Apple_Boot Recovery HD             650.1 MB   disk1s3
   4:                  Apple_HFS Data                    326.8 GB   disk1s4
/dev/disk2
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS Macintosh HD           *2.8 TB     disk2
                                 Logical Volume on disk0s2, disk1s2
                                 03BC8817-F05E-4FC3-9A5C-50130CDAB3AB
                                 Unencrypted Fusion Drive
Your internal disk is a Fusion Drive combining an SSD (disk0) and a HDD (disk1).
It uses a Core Storage partition scheme.

The superior "Macintosh HD" is the name of the Logical Volume Group build by the compound of the two Physical Volumes (disk0s2/disk1s2) of your Fusion Drive.
It's used instead of the name of the physical device in a non-Core Storage partition layout.

The first inferior "Macintosh HD" in black is the name of the Logical Volume (disk2) as well as the HFS volume.

The two inferior "Macintosh HD" in grey are the "names" of the single Physical Volumes (disk0s2/disk1s2) contributing to the Logical Volume Group.

To get the all necessary informations about your Core Storage Volume Group(s) just enter diskutil cs list in Terminal.app.

Example:

CoreStorage logical volume groups (1 found)
|
+-- Logical Volume Group 47F9D6B1-F8F2-4E64-8AD4-9F2E2BD78E29
    =========================================================
    Name:         Macintosh HD
    Status:       Online
    Size:         3120722075648 B (3.1 TB)
    Free Space:   114688 B (114.7 KB)
    |
    +-&lt; Physical Volume ABF9D6B1-F8F2-4E64-8AD4-9F2E2BD78E29
    |   ----------------------------------------------------
    |   Index:    0
    |   Disk:     disk0s2
    |   Status:   Online
    |   Size:     120988852224 B (121.0 GB)
    |
    +-&lt; Physical Volume DE0BE70D-F8F2-4E64-8AD4-994F887A7BC1
    |   ----------------------------------------------------
    |   Index:    1
    |   Disk:     disk1s2
    |   Status:   Online
    |   Size:     2999733223424 B (3.0 TB)
    |
    +-&gt; Logical Volume Family 63FFE3F6-F9FE-48A6-923A-994F887A7BC1
        ----------------------------------------------------------
        Encryption Status:       Unlocked
        Encryption Type:         None
        Conversion Status:       NoConversion
        Conversion Direction:    -none-
        Has Encrypted Extents:   No
        Fully Secure:            No
        Passphrase Required:     No
        |
        +-&gt; Logical Volume FC7B83AA-E787-4E65-8C7E-2A62CDBB948C
            ---------------------------------------------------
            Disk:                  disk2
            Status:                Online
            Size (Total):          3106191572992 B (3.1 TB)
            Conversion Progress:   -none-
            Revertible:            No
            LV Name:               Macintosh HD
            Volume Name:           Macintosh HD
            Content Hint:          Apple_HFS


The two EFI partitions on your two physical devices are necessary and may contain the boot loader programs for some installed operating systems (e.g. rEFInd). Additionally the EFI partition is used as a staging area for (Apple-) firmware updates.