Creating a partition in unallocated space

Question

I left some unallocated space on my external HD (around 200 GB) and now I would like to create a partition on it using gpt in terminal since the Disk Utility tool on El Capitan is not even showing the free space, though it did show on Yosemite, but nothing would happen after clicking apply. Does it look ok? Which line indicates the free space? Thank you!

sudo gpt -r show disk2
disk2: Suspicious MBR at sector 0
       start        size  index  contents
           0           1         MBR
           1           1         Pri GPT header
           2          32         Pri GPT table
          34           6         
          40      409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
      409640   585937504      2  GPT part - 53746F72-6167-11AA-AA11-00306543ECAC
   586347144      262144      3  GPT part - 426F6F74-0000-11AA-AA11-00306543ECAC
   586609288        1400         
   586610688   976562176      4  GPT part - EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
  1563172864   390352270         
  1953525134          32         Sec GPT table
  1953525166           1         Sec GPT header

diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *121.3 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:          Apple_CoreStorage Macintosh HD            120.5 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
/dev/disk1 (internal, virtual):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS Macintosh HD           +120.1 GB   disk1
                             Logical Volume on disk0s2
                             A0CB7732-E8FC-48BB-B3DB-919A1F0E731E
                             Unlocked Encrypted
/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:          Apple_CoreStorage Mac Backup 2            300.0 GB   disk2s2
   3:                 Apple_Boot Boot OS X               134.2 MB   disk2s3
   4:       Microsoft Basic Data FAT                     500.0 GB   disk2s4
/dev/disk3 (external, virtual):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS Mac Backup 2           +299.6 GB   disk3
                             Logical Volume on disk2s2
                             95D2FFB0-FDB0-4133-A407-849A654D82A6
                             Unlocked Encrypted

diskutil info /dev/disk2
   Device Identifier:        disk2
   Device Node:              /dev/disk2
   Whole:                    Yes
   Part of Whole:            disk2
   Device / Media Name:      Seagate BUP Slim Mac SL Media

   Volume Name:              Not applicable (no file system)

   Mounted:                  Not applicable (no file system)

   File System:              None

   Content (IOContent):      GUID_partition_scheme
   OS Can Be Installed:      No
   Media Type:               Generic
   Protocol:                 USB
   SMART Status:             Not Supported

   Total Size:               1.0 TB (1000204885504 Bytes) (exactly 1953525167 512-Byte-Units)
   Volume Free Space:        Not applicable (no file system)
   Device Block Size:        512 Bytes

   Read-Only Media:          No
   Read-Only Volume:         Not applicable (no file system)

   Device Location:          External
   Removable Media:          No

   Virtual:                  No
   OS 9 Drivers:             No
   Low Level Format:         Not supported
sudo fdisk /dev/disk2
Disk: /dev/disk2    geometry: 121601/255/63 [1953525167 sectors]
Signature: 0xAA55
         Starting       Ending
 #: id  cyl  hd sec -  cyl  hd sec [     start -       size]
------------------------------------------------------------------------
 1: EE 1023 254  63 - 1023 254  63 [         1 -     409639] <Unknown ID>
 2: AC 1023 254  63 - 1023 254  63 [    409640 -  585937504] <Unknown ID>
 3: AB 1023 254  63 - 1023 254  63 [ 586347144 -     262144] Darwin Boot 
 4: 0B 1023 254  63 - 1023 254  63 [ 586610688 -  976562176] Win95 FAT-32

Answer 1

The empty space is on disk2 right behind your FAT32 partition (index number 4) with the start block 1563172864 and the size 390352270 blocks (visible in your sudo gpt -r show disk2 output right at the beginning of your listings).

First backup all your volumes.

To create a new volume with gpt boot from a different disk (e.g. disk0s2/disk1: "Macintosh HD").

After booting open Terminal.app.

Enter diskutil list to get an overview.

Then identify the DiskIdentifier (e.g disk2) of the disk you want to add a volume to and enter sudo gpt -r show /dev/disk2 to get the partition table of disk2.

Unmount all disks related to disk2. This is disk2 itself and any inlying and mounted CoreStorage volume (e.g. disk3 aka "Mac Backup 2") on this disk:

diskutil umountDisk /dev/disk3
diskutil umountDisk /dev/disk2

To create a new partition enter:

sudo gpt add -b StartBlock -i IndexNumber -s SizeInBlocks -t GUIDType /dev/disk2

StartBlock is the number of the first block in the unallocated space and NumberOfBlocks is the number of free blocks or smaller. NumberOfBlocks has to be dividable through 8!. As IndexNumber use (biggest index number already there + 1). Then enter a GUIDType.

One valid command would be:

sudo gpt add -b 1563172864 -i 5 -s 390352264 -t 48465300-0000-11AA-AA11-00306543ECAC /dev/disk2

If you want to leave a gap of 100 MiB between your Windows partition and the new volume use the following command instead:

sudo gpt add -b 1563377664 -i 5 -s 390147464 -t 48465300-0000-11AA-AA11-00306543ECAC /dev/disk2

Quit Terminal and open Disk Utility. Erase the newly created partition, format the volume (JHFS+) and rename it.