re expand a partitioned disk

The internal disk of my Mac book pro is partitioned as follows:

% diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.3 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS mainHD                  249.7 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
   4:          Apple_CoreStorage                         249.1 GB   disk0s4
   5:                 Apple_Boot Recovery HD             650.0 MB   disk0s5
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS oldHD                  *248.7 GB   disk1


% diskutil cs list
CoreStorage logical volume groups (1 found)
|
+-- Logical Volume Group C2BDDBCA-73CF-473C-B9DA-E5682EF29610
    =========================================================
    Name:         Macintosh HD 2
    Status:       Online
    Size:         249059012608 B (249.1 GB)
    Free Space:   35708928 B (35.7 MB)
    |
    +-< Physical Volume CA1B7050-0A3D-4FFD-ADC2-AE42B45BEB9C
    |   ----------------------------------------------------
    |   Index:    0
    |   Disk:     disk0s4
    |   Status:   Online
    |   Size:     249059012608 B (249.1 GB)
    |
    +-> Logical Volume Family 4CBD3D40-147F-4B2D-A997-975E00295101
        ----------------------------------------------------------
        Encryption Status:       Unlocked
        Encryption Type:         None
        Conversion Status:       NoConversion
        Conversion Direction:    -none-
        Has Encrypted Extents:   No
        Fully Secure:            No
        Passphrase Required:     No
        |
        +-> Logical Volume B3E1FCD6-ACAB-4152-AEB5-71F41C329C17
            ---------------------------------------------------
            Disk:                  disk1
            Status:                Online
            Size (Total):          248687755264 B (248.7 GB)
            Conversion Progress:   -none-
            Revertible:            Yes (no decryption required)
            LV Name:               oldHD
            Volume Name:           oldHD
            Content Hint:          Apple_HFS

The operating system (os x 10.9.5) and all my files are in the mainHD, where I'm running out of space. I would like to delete the oldHD so that I can increase the size of mainHD.

When I try to delete oldHD via the graphical disk utility I get an Alert (null) box where I can select cancel or remove, if I click on remove nothing happens and in the log I see:

: Preparing to remove partition from disk: “APPLE SSD SM512E Media”
: Could not unmount partition to be deleted.

How can I get oldHD deleted and mainHD resized?