Unable to access neither Partition Layout nor Option tab in Partition tab on Disk Utility I have a problem that I cannot select Option tab in Partition tab as shown in above screenshot. I wanted to convert my external hard disk to GUID partition table.

Thank you in advance for your help.
You can't partition disk1 freely because it contains a Core Storage Volume Group (disk1s2 -> disk2).

You can revert to a non-CoreStorage by booting to Macintosh HD (disk0s2) and opening Terminal.

Enter the following to get all necessary infos:

diskutil cs list


Example listing:

      CoreStorage logical volume groups (1 found)
    |
    +-- Logical Volume Group A629E051-D7B0-4B8C-A803-074F62704636
        =========================================================
        Name:         System
        Status:       Online
        Size:         53946696192 B (53.9 GB)
        Free Space:   16777216 B (16.8 MB)
        |
        +-&lt; Physical Volume 90C09FC0-4215-4871-901B-70E2C9C7D464
        |   ----------------------------------------------------
        |   Index:    0
        |   Disk:     disk0s2
        |   Status:   Online
        |   Size:     53946696192 B (53.9 GB)
        |
        +-&gt; Logical Volume Family F6962E38-50E4-4458-BFE6-CF2E179352F5
            ----------------------------------------------------------
            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 BD36C73D-860D-4DC6-B125-AD624F448B88
                ---------------------------------------------------
                Disk:                  disk2
                Status:                Online
                Size (Total):          53496696192 B (53.5 GB)
                Conversion Progress:   -none-
                Revertible:            Yes (no decryption required)
                LV Name:               System
                Volume Name:           System
                Content Hint:          Apple_HFS


If your Logical Volume is revertible enter:

diskutil cs revert lvUUID


(lvUUID is the UUID of the Logical Volume)

with the UUID of the example above

diskutil cs revert BD36C73D-860D-4DC6-B125-AD624F448B88


If the Logical Volume is encrypted you have to enter a passphrase. 

After the conversion repartition the drive as needed.



If the LV isn't revertible you have to copy the content of the volume 21Tech to another volume and delete the Logical Volume Group with:

diskutil cs delete lvgUUID


(lvgUUID is the UUID of the Logical Volume Group)

with the UUID of the example above

diskutil cs delete A629E051-D7B0-4B8C-A803-074F62704636


Then repartition the drive as needed.